Obama has said repeatedly that the use of chemical weapons would cross a "red line" and constitute a "game changer" for U.S. policy on Syria, which until now has focused entirely on providing the opposition with nonlethal assistance and humanitarian aid.
The White House said it had notified Congress, the United Nations and key international allies about the new U.S. chemical weapons determination. Obama will discuss the assessments, along with broader problems in Syria, during the summit of eight leading industrial nations next week in Northern Ireland.
Among those in attendance will be Russian President Vladimir Putin, one of Assad's most powerful backers. Obama and Putin will hold a one-on-one meeting on the sidelines of the summit, and the U.S. leader is expected to press his Russian counterpart to drop his political and military support for the Syrian government.
But Putin's foreign affairs adviser, Yuri Ushakov, said Friday that Moscow had doubts about Washington's claim Assad had used chemical weapons against the opposition.
He told reporters the information provided by U.S. officials to Russia "didn't look convincing."
The Syrian fighters have been clamoring for bolder Western intervention, particularly given the estimated 5,000 Hezbollah guerrillas propping up Assad's forces. Assad's stunning military success last week at Qusair, near the Lebanese border, and preparations for offensives against Homs and Aleppo have made the matter more urgent.
Rep. Ed Royce, R-Calif., the chairman of the House Foreign Affairs Committee, said he supported the president's decision "to expand assistance for the vetted Syrian opposition." But other lawmakers expressed reservations, including Sen. Chris Murphy, D-Conn., a member of the Senate Foreign Relations Committee.