The US knows little about chemical weapon use in Syria, president Barack Obama has admitted.
He said he did not know how or when they were used or who used them. He told a news conference he had to make sure of the facts before deciding how to respond to evidence that chemical weapons were used and he has asked the Pentagon for range of options if president Bahsar Assad has used them.
The president said, however, that if it is determined that the Assad regime used chemical weapons "we would have to rethink the range of options that are available to us."
With the US disengaging from the unpopular war in Afghanistan and still smarting from the difficult conflict in Iraq, Mr Obama has been reticent to unleash American military power in the Syrian fighting, a civil war that has killed tens of thousands of people, something that he said was a "blemish on the international community generally."
But he said he was not prepared to rush to respond to growing evidence that chemical weapons had been used in Syria, something he termed would mark the crossing of a "red line" and a game-changer, Mr Obama said: "I meant that we would have to rethink the range of options open to us."
In the White House news conference marking the 100th day of his second term, the president said he had a full range of such "options on the shelf" but he declined to list them.
Many critics of Mr Obama's reluctance to use the American military in Syria are calling for the president to establish safe zones for Syrian rebels, to protect them with a no-fly zone and begin sending arms to forces fighting to overthrow the Assad regime.
The problem is that Syrian air defences are far stronger than Nato allies faced when they intervened with air power in Libya, and many of the rebel forces are now identified as Islamic radicals, many of them associated with al Qaida and determined to establish a government based on a strict interpretation of Islamic law.
Noting American humanitarian aid that has flowed into victims of the conflict, Mr Obama said the civil war has been "a slowing unfolding disaster for the Syrian people, and this is not a situation in which we've simply been bystanders."
But when measuring additional action, Obama said, "I've got to know I've got the facts." He further declared that the international community has to be completely confident in the assessment that chemical weapons have been used. "We don't know who used them. We don't have a chain of custody that establishes" exactly what happened.