United Nations inspectors have taken blood samples from the victims of last week's alleged chemical weapons attack by the Syrian military.
The UN team Monday visited one of several Damascus suburbs where rebels claim the government unleashed poison gas, killing hundreds of civilians, including entire families in their homes.
At one point, the UN team came under sniper fire that damaged a car and forcing the inspectors to turn back. No one was hurt. The rebels and government blame each other for the gunfire.
UN spokesman Farhan Haq said the inspectors were satisfied with their meetings with doctors and victims, and will visit another attack site Tuesday.
But a top U.S. official says any evidence of a poison gas attack may be corrupted because the Syrian government kept the team from visiting the sites for five days.
U.S. and European leaders are discussing how to respond. U.S. Defense Secretary Chuck Hagel said any action will be taken "in concert with the international community" and within the law.
But Russian Foreign Minister Sergei Lavrov, a major Syrian ally, says the West cannot find any evidence that the Syrian government used chemical weapons. Lavrov accuses the West of "hysteria" and warned that any military action would be a "gross violation of international law."
"All those in Syria have a stake in finding out the truth," said UN Secretary-General Ban Ki-moon. "The whole world should be concerned about any threat or use of chemical weapons, and that is why the world is watching Syria."