BANGKOK, July 21 -- The Obama administration is increasingly concerned that nuclear-armed North Korea is building mysterious military ties with Burma, another opaque country with a history of oppression, Secretary of State Hillary Rodham Clinton said Tuesday.

"We know that there are also growing concerns about military cooperation between North Korea and Burma, which we take seriously," Clinton told reporters after talks in the Thai capital. "It would be destabilizing for the region. It would pose a direct threat to Burma's neighbors."

U.S. officials traveling with Clinton, who is in Thailand to attend a regional security forum, said the worries about Burma and North Korea extend to possible nuclear cooperation. North Korea has a long history of illicit missile sales and proliferation, including secretly helping to build a nuclear reactor in Syria that was destroyed in 2007 by Israeli jets.

The evidence of growing Burmese-North Korean cooperation since formal ties were restored in 2007 is extensive, but officials said the full scope of the military relationship is unclear.

The nuclear connection is even murkier, but intelligence agencies have tracked the suspicious procurement of high-precision equipment from Europe, as well as the arrival in Burma of North Korean officials associated with the company connected to the Syrian reactor, said David Albright, director of the Institute for Science and International Security in Washington.

"Something may be going on, but no one has any proof. It is a mix of suspicions and concerns," Albright said, adding that close examination of satellite imagery of suspected nuclear sites has turned up no evidence. But he said that the purchases of high-precision equipment were especially troubling because the equipment did not make sense for use in missiles and it was shipped to educational entities that had connections to Burmese nuclear experts.

Japanese officials last month arrested three people in connection with attempting to illegally export dual-use equipment to Burma via Malaysia, under the direction of a company involved in the illicit procurement for North Korean military programs.

Moreover, Albright said, European and U.S. intelligence agencies have identified people associated with Namchongang Trading, a North Korean company also known as NCG, as working in Burma. NCG reportedly provided the critical link between Pyongyang and Damascus, acquiring key materials from vendors in China and probably Europe and secretly transferring them to a desert construction site near the Syrian town of Kibar.

The State Department cited NCG last month for being "involved in the purchase of aluminum tubes and other equipment specifically suitable for a uranium enrichment program since the late 1990s."

U.S. officials have observed other troubling connections. The Navy tracked Kang Nam 1, a rusty North Korean freighter, last month after the government in Pyongyang tested a nuclear weapon. Although U.S. officials were never certain that the ship was headed to Burma, it returned to North Korea after the United States, China and other countries pressured Burma to respect a U.N. resolution barring most North Korean weapons exports.

Photographs that have emerged in recent weeks also show an extensive series of 600 to 800 tunnel complexes and other underground facilities built in Burma with North Korean technical assistance near its new capital, Naypyidaw. North Korean officials can be spotted in the photos, which were taken between 2003 and 2006 and posted on the Web site of YaleGlobal Online by journalist Bertil Lintner, an expert on Burma.

Burma has uranium deposits, but as a signatory to the nuclear Non-Proliferation Treaty, it is required to allow inspections of any nuclear facilities. Russia agreed in 2007 to help build a 10-megawatt light-water reactor in Burma, but little appears to have come of the project.

At the news conference, Clinton also strongly criticized the Burmese government for its well-documented use of gang rape as a military tactic, organized by Burmese officers, against ethnic minorities. A new offensive against the Karen ethnic group has sent more than 4,000 refugees fleeing across the border into Thailand in recent weeks.

"We are deeply concerned by reports of continuing human rights abuses within Burma, particularly by actions that are attributed to the Burmese military concerning the mistreatment and abuse of young girls," Clinton said.