* Obama voices "grave concern", avoids pledging action
* Activists say sending body tissue, photos to U.N. team
* Russia urges Assad to cooperate with inspectors
* China says must not prejudge outcome of probe
* Divisions at the Security Council weaken response
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 to U.N. inspectors from victims of Wednesday's mass poisoning, Obama brushed over an interviewer's reminder that he once called chemical weapons a "red line" that could trigger U.S. action.
A White House spokesman reiterated Obama's position that he did not expect to have "boots on the ground" in Syria.
Obama's caution contrasted with calls for action from NATO allies, including France, Britain and Turkey, where leaders saw little doubt 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 Assad of a cover-up by preventing the U.N. team from visiting the scene, Moscow said the rebels were impeding an investigation.
The United Nations 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.
Obama played down the chances of Assad cooperating with the U.N. experts who might provide conclusive evidence of what happened, if given access soon.
Noting budget constraints, problems of international law and a continuing U.S. casualty toll in Afghanistan, 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 United States, he replied: "If the U.S. goes in and attacks another country without a U.N. 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 United Nations Security Council moves against Assad in the past and oppose military action.
Having abstained to allow NATO powers a U.N. mandate to back Libyan rebels against 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 U.S.-led invasion of Iraq in 2003, despite a failure to secure a specific U.N. mandate for it, led to long wrangling over whether Washington and its allies broke 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 Assad to cooperate with the U.N. inspection team which 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.
U.N. Secretary General Ban Ki-moon said he intends to conduct a "thorough, impartial and prompt investigation" into the latest allegations. Top U.N. disarmament official Angela Kane was due to arrive in Damascus on Saturday to push for access to the site for the U.N. 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," 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 U.N. team to go there?" he added, saying that the attack was "not something that a humane and civilised world" could ignore.
But Russia, 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 U.N. inspectors.
"We will ensure the safety of the U.N. team ... It is critical that those inspectors get there within 48 hours," 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 U.N. 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 U.N. 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 U.N. 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 U.N. 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 U.S. official familiar with initial intelligence assessments said on Thursday that the gassing appeared to be the work of the Assad government.
Images, including some by freelance photographers supplied to Reuters, showed scores of bodies laid out on floors with no visible signs of injury. Some had foam at the nose and mouth.
CALLS FOR ACTION
French Foreign Minister Laurent Fabius said on Thursday that world powers must respond with force if allegations that Syria's government was responsible for the deadliest chemical attack on civilians in a quarter-century prove true. Fabius stressed, however, there was no question of sending in troops.
European officials said that options ranging from air strikes or a no-fly zone to providing heavy weapons to some rebels were all still on the table. But there was little prospect of concrete measures without U.S. backing. "Without U.S. firepower, there's little we can do," one said.
A senior parliamentary ally of British Prime Minister David Cameron said: "So long as the Americans are of the view that nothing should be done, I don't see the British government pressing them ... And we're not capable of doing anything without them .. There's absolutely no appetite from any party for intervention anywhere."
Turkey, fearful of instability on its long southern border, called for an end to talk and time-wasting. "There is nothing left to say now," said President Abdullah Gul. "It is now time for actual concrete action ... The price of playing down the events and procrastinating through diplomatic manoeuvring and trickery in the U.N. Security Council will be very high."
Obama has asked U.S. intelligence agencies to help establish what caused the deaths, a State Department spokeswoman said on Thursday. But he has set no deadline.
The State Department also said U.S. and Russian diplomats would discuss Syria at The Hague next Wednesday, continuing contacts that have so far produced little to end the conflict.