Obama considering limited military strike on Syria UN chemical weapons experts meet residents at one of the sites of an alleged poison gas attack in the Damascus suburb of Mouadamiya. US mood darkens over Syria: Nick O'Malley Washington: President Barack Obama is weighing a military strike against Syria that would be of limited scope and duration, designed to serve as punishment for Syria's use of chemical weapons and as a deterrent, while keeping the United States out of deeper involvement in that country's civil war, according to senior administration officials. The timing of such an attack, which would probably last no more than two days and involve sea-launched cruise missiles - or, possibly, long-range bombers - striking military targets not directly related to Syria's chemical weapons arsenal, is dependent on three factors: completion of an intelligence report assessing Syrian government culpability in last week's alleged chemical attack; ongoing consultation with allies and Congress; and determination of a justification under international law. "Nothing today is more serious, and nothing is receiving more serious scrutiny": US Secretary of State John Kerry on Monday afternoon. "We're actively looking at the various legal angles that would inform a decision," said an official who spoke about the presidential deliberations on the condition of anonymity. Missile-armed US warships are already positioned in the Mediterranean. As the administration moved rapidly towards a decision, the US Secretary of State, John Kerry, said the use of chemical weapons in an attack on Wednesday against opposition strongholds on the eastern outskirts of Damascus is now "undeniable." Evidence being gathered by United Nations experts in Syria was important, Kerry said, but not necessary to prove what is already "grounded in facts, informed by conscience and guided by common sense." The team of UN weapons investigators on Monday visited one of three rebel-held suburbs where the alleged attack took place, after first being forced to withdraw when their vehicles came under sniper fire. The Syrian government, which along with Russia has suggested that the rebels were responsible for the chemical attack, agreed to the UN inspection over the weekend. Videos and statements by witnesses and relief organisations such as Doctors Without Borders have proven that an attack occurred, Kerry said. The US intelligence report is to be released this week. Among the factors, officials said, are that only the government is known to possess chemical weapons and the rockets to deliver them, and its continuing control of chemical stocks has been closely monitored by US intelligence. Kerry said Syrian forces had engaged in a "cynical attempt to cover up" their actions, not only by delaying the arrival of the UN team but by shelling the affected area continually. Any strike would probably await the departure of the UN inspectors from Syria. Kerry's statement, which he read to reporters in the State Department briefing room without taking questions, was part of an escalating administration drumbeat, which is likely to include a public statement by Obama in coming days. Officials said the public warnings are designed partly to wring any possibility of cooperation out of Russia - or an unlikely admission from the government of Syrian President Bashar Assad - before Obama makes his decision. Make no mistake: President Obama believes there must be accountability for those who would use the world's most heinous weapons against the world's most vulnerable people. Nothing today is more serious, and nothing is receiving more serious scrutiny," Kerry said. He and other officials drew a sharp distinction between US action related to a violation of international law by what they called Assad's "massive" use of chemical weapons and any direct military involvement in the Syrian conflict, which is in its third year. Obama and other officials have said repeatedly that no US troops would be sent to Syria. But despite Obama's year-old threat of an unspecified US response if Assad crossed a "red line" by using chemical weapons, even a limited military engagement seemed unlikely before Wednesday's attack near Damascus. "This international norm cannot be violated without consequences," Kerry said. The options under consideration are neither new nor open-ended, officials said. The use of "limited stand-off strikes" has long been among the options the Pentagon has provided Obama. "Potential targets include high-value regime air defence, air, ground, missile and naval forces as well as the supporting military facilities and command nodes," General Martin Dempsey, chairman of the Joint Chiefs of Staff, said in a June letter to Congress. Stand-off air and missile systems could be used to strike hundreds of targets at a tempo of our choosing. Although Dempsey, who has questioned the wisdom of direct military involvement in Syria, said that such an operation would require "hundreds" of ships and aircraft and potentially cost "in the billions," the action that is being contemplated would be far smaller and designed more to send a message than cripple Assad's military and change the balance of forces on the ground. Syrian chemical weapons storage areas, which are numerous and widely dispersed, are seen as unlikely targets. The language of international criminality has clearly resonated among US allies and lawmakers. "We will have to act," said Adam Schiff, a member of the House Permanent Select Committee on Intelligence who has long opposed any US intervention, including the administration's decision earlier this summer to send light arms to Syrian opposition military forces. I don't think we can allow repeated use of chemical weapons now, an escalated use of chemical weapons, to stand. Senator Bob Corker, the senior Republican on the Foreign Relations Committee, emphasised that a US strike should not be directed at altering the dynamic of Syria's larger civil war. I think it should be surgical. It should be proportional. It should be in response to what's happened with the chemicals," Corker said in an NBC interview. But the fact is, I don't want us to get involved in such a way that we change that dynamic on the ground. The senator said he thought the administration's response to the attack is "imminent." House Speaker John Boehner said he had been in touch with the White House. In a statement, Boehner echoed concerns expressed by lawmakers from both parties that the administration further consults Congress before taking action. The administration has said that it will follow international law in shaping its response. Authorisation for the use of force against another nation normally comes only from the UN Security Council - where Russia and China have vetoed previous resolutions against Assad - or in a NATO operation similar to the one launched in the former Yugoslavia in 1999, without a UN mandate. But much of international law in untested, and administration lawyers are also examining possible legal justifications based on a violation of international prohibitions on chemical weapons use, or on an appeal for assistance from a neighbouring nation such as Turkey. Britain, France and Turkey have said that they would support action if the use of chemical weapons is confirmed, but a clear-cut case is also likely to make approval easier for allies such as Germany, which disagreed with NATO's 2011 operation in Libya despite the existence of a UN resolution. "The use of chemical weapons would be a crime against civilisation," German Foreign Minister Guido Westerwelle said on Monday. The international community must act should the use of such weapons be confirmed. Consultations on Syria have been ongoing at the ambassadorial level at NATO headquarters in Brussels, where a meeting is scheduled for Wednesday. The Arab League, which had approved the Libya operation, is also due to meet this week to discuss Syria.