Sen. Dick Durbin of Illinois, a member of the Democratic leadership, was asked what should be done about the crossing of what the administration has called a red line. He said, "That's up to the commander in chief, but something has to be done."
And Sen. John McCain, R-Ariz., said, "I think it's pretty obvious that that red line has been crossed. Now I hope the administration will consider what we have been recommending now for over two years of this bloodletting and massacre and that is to provide a safe area for the opposition to operate, to establish a no-fly zone and provide weapons to people in the resistance who we trust."
The White House said the current intelligence assessments are based in part on "physiological samples." Officials wouldn't say specifically what information they are lacking in order to conclusively determine that Syrian President Bashar Assad's government used chemical weapons.
However, the White House letters emphasized a need for the completion of a stalled U.N. investigation.
But it's unclear whether U.N. inspectors will ever be able to conduct a full investigation in areas where there is the most evidence of chemical weapons use. The Syrian government has so far refused to allow the U.N. experts to go anywhere but Khan al-Assal, where Assad's government maintains the rebels used the deadly agents.
A senior administration official said the U.S. was consulting with allies and looking for other ways to confirm the intelligence assessments.
The officials commented only on condition of anonymity because they were not authorized to speak publicly.
Until Thursday, the U.S. had resisted joining a growing number of allied nations that claim to have evidence that Syrian leader Assad's government has deployed chemical weapons.
Last month, British and French ambassadors to the United Nations told Secretary General Ban Ki-moon that the government used chemical weapons near Aleppo, in Homs and possibly in the capital of Damascus. Pressure mounted on the U.S. this week when two key allies in the Middle East -- Israel and Qatar -- also said there was evidence that Assad had used chemical weapons.
Following the U.S. disclosure, NATO spokeswoman Oana Lungescu said, "There would doubtlessly be a very strong reaction from the international community if there were evidence that chemical weapons had been used."
The White House, in its letters to Capitol Hill, said that "because the president takes this issue so seriously, we have an obligation to fully investigate any and all evidence of chemical weapons use within Syria."
The letters were sent to McCain and to Sen. Carl Levin, D-Mich.
The letters also said the U.S. believes the use of chemical weapons "originated with the Assad regime." That is consistent with the Obama administration's assertion that the Syrian rebels do not have access to the country's stockpiles.