U.N. chemical inspectors toured the stricken areas last week, collecting biological and soil samples, but it is not clear when they will present their findings.
The Obama administration has failed to bring together a broad international coalition in support of military action, having so far only secured the support of France.
Britain's parliament narrowly voted against British participation in a military strike last week, despite appeals by Prime Minister David Cameron, and the Arab League has stopped short of endorsing a Western strike against Syria.
In an emergency meeting on Sunday, the 22-state League called on the United Nations and the international community to take "deterrent" measures under international law to stop the Syrian regime's crimes, but could not agree on whether to back U.S. military strikes.
Russia or China would likely veto any U.N. Security Council resolution sanctioning a Western strike against Syria.
China is "highly concerned" about possible unilateral military action against Syria and believes the international community must "avoid complicating the Syrian issue and dragging the Middle East down into further disaster," Foreign Ministry spokesman Hong Lei said Monday.
In Washington, the Obama administration was lobbying to secure domestic support.
Obama met Monday with former political rival Sen. John McCain at the White House, hoping the foreign policy hawk will help sell the idea of U.S. military intervention.
On Capitol Hill, senior administration officials briefed lawmakers in private on Sunday to explain why the U.S. was compelled to act against Assad. Further meetings were planned through Wednesday.
The Syrian conflict erupted in March 2011 as an uprising against Assad that later degenerated into a civil war. More than 100,000 Syrians have been killed in the conflict.
In Damascus, the Syria representative of the U.N. refugee agency, Tarik Kurdi, said that five million Syrians have been displaced inside the country by the war.
In addition, nearly 2 million Syrians have fled to neighboring countries, according to previous U.N. figures, bringing the total number of uprooted Syrians to about 7 million, or nearly one-third the country's estimated population of 23 million.
Kurdi said the need for aid is far greater than what the international community has provided so far.
"Whatever efforts we have exerted and whatever the U.N. has provided in humanitarian aid, it is only a drop in the sea of humanitarian needs in Syria," he told The Associated Press. The funding gap "is very, very wide," he added.