Meanwhile, the latest concerns about chemical weapons in Syria prompted a meeting of Obama's national security team. Secretary of State John Kerry, who is on vacation, attended via video teleconference and made a flurry of telephone calls to world leaders to discuss the unrelenting bloodshed in Syria.
The United States said in June that it had conclusive evidence that Assad's government had used chemical weapons against opposition forces. That crossed what Obama had called a "red line" and prompted a U.S. decision to send arms to Syrian rebels, including guns, ammunition and shoulder-fired, anti-tank grenades.
State Department spokeswoman Jen Psaki said that 1,000 to 1,800 people were killed near Damascus in the latest alleged chemical weapons attack. She said Obama had directed U.S. intelligence agencies to gather additional information, but "right now, we are unable to conclusively determine chemical weapons use."
French Foreign Minister Laurent Fabius on Tuesday raised the possibility of the international community using force. Turkish Foreign Minister Ahmet Davutoglu said at a news conference in Berlin, "Several red lines have been crossed — if sanctions are not imposed immediately, then we will lose our power to deter."
Kerry has also urged other nations to help gather information about the reports of chemical weapons attacks. And the Obama administration is expected to hold more meetings on the matter in coming days.
Sen. John McCain, R-Ariz., a leading advocate of a more aggressive U.S. response to the events in Syria, said on CNN that if Assad believes there will be no retaliation, Assad would see that "the word of the president of the United States can no longer be taken seriously."
Top military leaders have cautioned against even limited action in Syria. Dempsey, the Joint Chiefs chairman, said in a letter this week to a congressman that the U.S. military is clearly capable of taking out Assad's air force and shifting the balance of the war toward the armed opposition. But such an approach would plunge the U.S. into the war without offering any strategy for ending what has become a sectarian fight, he added.