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.
Associated Press writers Donna Cassata, Lara Jakes, Bradley Klapper and Lauran Neergaard contributed to this report. Burns reported from Abu Dhabi, United Arab Emirates.