Washington (AFP) - It's a Hollywood sci-fi fantasy that has long eluded the Pentagon: a flying "mothership" that launches smaller aircraft. 
 
 The Pentagon's research agency put out a request to industry this month to outline how a large cargo plane could release drones to spy on or attack an enemy and then return to the flying aircraft carrier. 
 
 The concept conjures up fantastical images from "The Avengers" film and the "StarCraft" video game, with large, lumbering motherships sending out smaller craft -- but there are no cloaking devices involved in this particular project. 
 
 At the moment, the Defense Advanced Research Projects Agency (DARPA) is not close to constructing any test planes and is merely exploring the possibility on paper, officials and experts said. 
 
 "This is the float-the-idea stage of the concept. It's not the build-me-a-prototype stage. We're not to that yet," said author Peter Singer, who has written extensively about robots and warfare. 
 
 In a conceptual drawing from DARPA, a cargo plane resembling a C-130 releases a squadron of drones that look similar to Predator or Reaper aircraft. 
 
 "We want to find ways to make smaller aircraft more effective, and one promising idea is enabling existing large aircraft, with minimal modification, to become 'aircraft carriers in the sky,'" Dan Patt, the manager for the DARPA program, said in a statement. 
 
 - Airships with biplanes - 
 
 It's not the first time the American military has sought to create a carrier in the sky. 
 
 "This idea goes back to the 1920s," said James Lewis, director of the Strategic Technologies Program at the Center for Strategic and International Studies. 
 
 "If you think back to dirigibles, they used to have small aircraft, a one-man fighter, that would hook up to them." 
 
 Constructed by the US Navy in the late 1920s, the rigid airships could carry a small squadron of Sparrowhawk biplanes inside, and the planes would then launch from the dirigible after being lowered by a trapeze device. The biplane would return by latching onto a hook on the belly of the mothership. 
 
 The US Navy built two of the airships but both of them went down in disastrous crashes in the 1930s, killing dozens of crew and spelling the end of the experiment. 
 
 In the 1960s, the CIA commissioned a pioneering drone manufactured in secret by Lockheed Martin, the D-21, which was designed to launch from another jet and later from a B-52 bomber. The unmanned D-21 carried a camera for spy missions over China and then was supposed to release the camera for retrieval while the plane would self-destruct. 
 
 In four missions, the plane either failed to self-destruct as planned or the camera module could not be retrieved, and the program was scrapped by 1971. 
 
 - Underwater aircraft carrier? - 
 
 The flying carrier could allow the use of drones in areas where the United States has no access to nearby airfields, but the technical challenge of recovering a robotic plane in mid-air remains a serious obstacle, Lewis said. 
 
 The exploration of a possible airborne carrier is part of a massive effort by the Pentagon to invest in cutting-edge robotic technology, as commanders are keenly aware that China, Russia, Iran and other countries are looking to build their own fleets of unmanned weapons. 
 
 The carrier fits into what researchers call the "marsupial" category of robots. At the recent Zhuhai arms show, China unveiled a prototype of an unmanned armored vehicle that carries two smaller robots inside, a tracked robot outfitted with a machine gun and a flying surveillance quadcopter. 
 
 But while the flying drone carrier may be a long way off, US military researchers are moving closer to an underwater carrier, with aircraft launched from a torpedo or missile tube, Singer said 
 
 "If you are looking at other places where you might see aircraft carriers, don't look up in the air, look under water," he said. "That's where there is some really interesting research work and strategic need." 
 
 Last year the US Naval Research Laboratory said it had successfully released a drone from a submerged submarine, with the robotic plane flying out of a Tomahawk missile tube and its wings unfolding origami style. ||||| This file aerial view taken on July 27, 2012, shows part of the city of Sansha on the island of Yongxing, also known as Woody island in the disputed Paracel chain, which China now considers part of Hainan province. (AFP/Getty) 
 
 A Chinese naval ship seized an underwater naval drone that was being used by the U.S. Navy to test water conditions in the South China Sea, the Pentagon said Friday. 
 
 Navy Capt. Jeff Davis, a Pentagon spokesman, said the incident occurred on Dec. 15 about 50 nautical miles northwest of Subic Bay, in international waters in the South China Sea. 
 
 The USNS Bowditch, an oceanographic survey vessel with a mostly civilian crew, was in the process of recovering two unmanned ocean gliders, which are used to collect information about water conditions that can help U.S. vessels operate. A Chinese ship, a Dalang-III class submarine rescue vessel, approached the area, coming within about 500 yards of the Bowditch before dropping a small boat in the water. It seized one of the gliders and brought it aboard, Davis said. 
 
 The Bowditch contacted the Chinese ship and asked for the glider to be returned. Officials aboard the Chinese ship acknowledged the radio communication, Davis said, but said they were returning to normal operations. The ship then left the area. 
 
 “We would like it back and we would like this not to happen again,” Davis said, referring to the underwater drone. The incident occurred around 1:45 p.m. local time, the Navy said. 
 
 
 
 
 
 A defense official, speaking on condition of anonymity because he was not authorized to discuss the matter on the record, said the United States has made an official diplomatic communication to the Chinese government, “demanding the return of our stuff.” 
 
 There was no immediate response from Beijing, the official said. 
 
 The incident comes amid mounting tensions over Beijing’s territorial claims in the South China Sea, which have alarmed U.S. allies in the region and prompted the U.S. military to conduct “freedom of navigation” operations intended as a show of force. This week, a U.S. think tank reported that China has placed antiaircraft weapons on artificial islands, threatening to intensify the debate over disputed areas. 
 
 Davis said ships such as the Bowditch routinely conduct operations in the South China Sea. The Navy said the drones are operated by the Naval Oceanographic Office, which states in its promotional materials that it maintains the largest glider fleet in the world, with more than 130 “littoral battlespace sensing” crafts. 
 
 The gliders are piloted by civilian employees of the oceanographic office from Stennis Space Center in Mississippi through the use of encrypted satellite communications. They typically travel just a few miles per hour and are tracked by oceanographic vessels such as the USNS Bowditch. The data the drones collect is unclassified. 
 
 Wu Shicun, president of China’s National Institute for South China Sea Studies, said the unmanned vehicle posed “a threat to China’s national security” because it could be used to “spy” on China’s land reclamation efforts or collect information about Chinese submarine routes. 
 
 “Why are [the underwater vehicles] there? I think the U.S. knows it very well,” he said. 
 
 Sen. John McCain, (R.-Ariz.), chairman of the Senate Armed Services Committee, said Friday afternoon that the seizure is a “flagrant violation” of the laws of the seas. 
 
 “China had no right to seize this vehicle,” McCain said. “And the United States must not stand for such outrageous conduct.” 
 
 McCain said the incident fits a pattern of increasingly destabilizing Chinese behavior. 
 
 “This behavior will continue until it is met with a strong and determined U.S. response, which until now the Obama administration has failed to provide,” McCain said. “Freedom of the seas and the principles of the rules-based order are not self-enforcing. American leadership is required in their defense.” 
 
 Chinese analysts, however, downplayed the incident. “It won’t have too much impact on the Sino-U.S. relationship,” said Zhu Feng, dean of the School of International Relations at Nanjing University. “China and the U.S. have been through conflicts far more severe.” 
 
 It’s not clear how President-elect Donald Trump, who has been the subject of Chinese government ire since his recent outreach to leadership of Taiwan, will approach those maritime disputes. But he has criticized Beijing for “building a massive fortress in the middle of the South China Sea,” in addition to his complaints about Chinese monetary and trade policies. 
 
 Patrick Cronin, an expert on Asia-Pacific security at the Center for a New American Security, called the seizure “a brazen, calculated act of coercive diplomacy” and said it was intended to send a message to Trump ahead of his inauguration. 
 
 “Rather than wait several weeks, Beijing is advancing a provocative action offshore from a U.S. ally that had recently kowtowed to China,” Cronin said, referring to the incident’s location near the Philippines. The signal China wishes to send is unambiguous, he said: “‘If you challenge our sovereignty we will challenge yours.’ The U.S. response needs to be equally clear: if anyone messes with with our Navy the response will not be limited to words.” 
 
 Both sides are merely signalling, said Qiao Liang, a military commentator and major general in the Chinese air force. “You called Tsai Ing-wen, sending a signal; we capture the UUV in South China Sea, our own exclusive economic zone,” he said. “It’s certainly a signal to the U.S.” 
 
 But, he added, “there will not be any escalation before Trump takes office.” 
 
 Luna Lin contributed from Beijing. 
 
 China has laid claim to a number of islands in the South China Sea, building airbases on tiny spits of land while installing powerful radar and missile launchers. Here's why. (Jason Aldag,Julie Vitkovskaya/The Washington Post / Satellite photos courtesy of CSIS) 
 
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 China puts new weapons on South China Sea islands, report says 
 
 Photos: China’s rapid island-building strategy continues 
 
 China testing Obama as it expands its influence in Southeast Asia
– In the latest effort to make reality out of what is currently science fiction, US officials are looking into the possibility of building a mothership: a large aircraft from which smaller ones take off. "This is the float-the-idea stage of the concept. It's not the build-me-a-prototype stage," says an expert. But this month, the Pentagon asked companies to envision designs for a drone-carrying cargo plane, AFP reports. The request seeks a design that could be demonstrated within four years, CNN reports. The effort has focused on converting aircraft the Pentagon already has. "One promising idea is enabling existing large aircraft, with minimal modification, to become 'aircraft carriers in the sky,'" says an official with the Defense Advanced Research Projects Agency. Versions of the idea have been around since the 1920s; in the 1930s, airships carrying Sparrowhawk biplanes crashed and the US dropped the effort for the time being. An underwater version of such a system could come sooner than a flying version, AFP notes: The US has already managed to launch a drone from a submarine's missile tube.