The more conclusive findings announced Thursday were aided by evidence sent to the United States by France, which, along with Britain, has announced it had determined that Assad's government had used chemical weapons.
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, next week during the G-8 summit 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, where the U.S. leader is expected to press his Russian counterpart to drop his political and military support for the Syrian government.
"We believe that Russia and all members of the international community should be concerned about the use of chemical weapons," Rhodes said.
Britain's U.N. Ambassador Mark Lyall Grant said his country was "not surprised by the determination made by the U.S. government," given its own assessments, and was in consultation with the Americans about next steps.
The U.S. has so far provided the Syrian rebel army with rations and medical supplies. In April, Kerry announced that the administration had agreed in principle to expand its military support to the opposition to include defensive items like night vision goggles, body armor and armored vehicles.
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.