BEIRUT (Reuters) - United States President Barack Obama called the apparent gassing of hundreds of Syrian civilians a "big event of grave concern", but stressed on Friday he was in no rush to embroil Americans in a costly new war.
As opponents of Syrian President Bashar al-Assad braved the frontlines around Damascus to smuggle tissue samples from victims of Wednesday's mass poisoning to the United Nations inspectors, Mr Obama brushed over an interviewer's reminder that he once called chemical weapons a "red line" that could trigger US action.
A White House spokesman reiterated Mr Obama's position that he did not expect to have "boots on the ground" in Syria.
Mr Obama's caution contrasted with calls for action from Nato allies, including France, Britain and Turkey, where leaders saw little doubt that Mr Assad's forces had staged pre-dawn missile strikes that rebels say killed between 500 and well over 1,000 people.
But two years into a civil war that has divided the Middle East along sectarian lines, a split between Western governments and Russia once again illustrated the international deadlock that has thwarted outside efforts to halt the killing.
While the West accused Mr Assad of a cover-up by preventing the UN team from visiting the scene, Moscow said the rebels were impeding an investigation.
The UN released data showing that a million children were among refugees forced to flee Syria, calling it a"shameful milestone". And mosque bombings that left at least 42 dead and hundreds wounded in neighbouring Lebanon were a reminder of how Syria's conflict has spread. But, for now, there seems little prospect of an end to the violence.
Mr Obama played down the chances of Mr Assad cooperating with the UN experts who might provide conclusive evidence of what happened, if given access soon.
Noting budget constraints, problems of international law and a continuing US casualty toll in Afghanistan, Mr Obama told CNN: "Sometimes what we've seen is that folks will call for immediate action, jumping into stuff that does not turn out well, gets us mired in very difficult situations, can result in us being drawn into very expensive, difficult, costly interventions that actually breed more resentment in the region.
"The United States continues to be the one country that people expect can do more than just simply protect their borders. But that does not mean that we have to get involved with everything immediately," he said, reflecting long-standing wariness to follow the example of his predecessor George W. Bush, and his ultimately unpopular ventures in Afghanistan and Iraq.
"We have to think through strategically what's going to be in our long-term national interests."
Asked about his comment - made a year and a day before the toxic fumes hit sleeping residents of rebel-held Damascus suburbs - that chemical weapons would be a "red line" for the US, he replied: "If the US goes in and attacks another country without a UN mandate and without clear evidence that can be presented, then there are questions in terms of whether international law supports it."
Russia and China have vetoed UN Security Council moves against Mr Assad in the past, and oppose military action.
Having abstained to allow Nato powers a UN mandate to back Libyan rebels against former Libyan strongman Muammar Gaddafi in 2011, Moscow and Beijing have closed ranks against what they see as a desire by Western states to change other countries' systems of government.
The US-led invasion of Iraq in 2003, despite a failure to secure a specific UN mandate for it, led to long wrangling over whether Washington and its allies had broken international law.
In June, Washington agreed in response to evidence of small chemical attacks to arm rebel groups, despite misgivings about Islamist radicals in their ranks, some allied with Al-Qaeda. But rebel leaders say it is too little, leaving only a stalemate.
International powers, including Moscow, have urged Mr Assad to cooperate with the UN inspection team that arrived on Sunday to pursue earlier allegations of chemical weapons attacks.
However, there was no public response from the Syrian government, whose forces have been pounding the region for days, making any mission by the international experts perilous - as well as possibly destroying evidence. Syria denies being responsible and has in the past accused rebels of using gas.
UN Secretary-General Ban Ki Moon said he intends to conduct a "thorough, impartial and prompt investigation" into the latest allegations. Top UN disarmament official Angela Kane was due to arrive in Damascus on Saturday to push for access to the site for the UN inspectors.
"I can think of no good reason why any party - either government or opposition forces - would decline this opportunity to get to the truth of the matter," Mr Ban said.
British Foreign Secretary William Hague said he believed the Syrian government was responsible for the casualties, which go on rising as medical staff and others fall sick. "It seems the Assad regime has something to hide," he said.
"Why else have they not allowed the UN team to go there?" he added, saying that the attack was "not something that a humane and civilised world" could ignore.
But Russia, Mr Assad's main arms supplier, said the opposition was preventing the objective investigation of what happened.
In an apparent rebuttal of that, Syria's opposition pledged to guarantee the safety of UN inspectors.
"We will ensure the safety of the UN team... It is critical that those inspectors get there within 48 hours," Mr Khaled Saleh, spokesman for the opposition Syrian National Coalition, told a news conference in Istanbul.
Opposition activists said they had been in contact with the specialist UN team in Damascus and had sent tissue samples with couriers trying to slip across from the Ghouta region into the government-held centre to deliver them to the inspectors.
Speaking from the town of Arbin, one of those affected by mysterious deaths from poisoning, opposition activist Abu Nidal told Reuters: "The UN team spoke with us and since then we prepared for them samples of hair, skin and blood and smuggled them back into Damascus with trusted couriers."
Activist Abu Mohammed, in Harasta, said: "We're being shelled and on top of that Ghouta is surrounded by regime checkpoints. But even that isn't a problem - we can smuggle them out. The problem is the location of the UN committee in the hotel. They're under heavy guard and government minders."
The rebels' efforts could prove futile; only material that has a clear provenance and a "chain of custody" would generally be treated as evidence by UN inspectors.
The longer the team waits for permission to investigate, the less likely it is to get to the bottom of an incident in which opponents say Syrian government forces fired rockets laden with poison gas canisters into rebel-held neighbourhoods.
A US official familiar with initial intelligence assessments said on Thursday that the gassing appeared to be the work of the Assad government.