"We've prepared for many contingencies in Syria," Rhodes said. "We are going to make decisions on further actions on our own timeline."
However, Sen. John McCain, R-Ariz., told reporters on Capitol Hill Thursday that he had been told Obama was "going to provide arms to the rebels."
Thursday's announcement followed a series of urgent meetings at the White House this week that revealed deep divisions within the administration over U.S. involvement in Syria's fierce civil war. While some State Department officials have been pressing for more robust action, advisers close to Obama are wary of sending weapons and ammunition into a war zone where Hezbollah and Iranian fighters are backing Assad's armed forces, and al-Qaida-linked extremists back the rebellion.
The White House said the Assad regime had used chemical weapons, including sarin, on a small scale multiple times in the last year. Up to 150 people have been killed in those attacks, the White House said, constituting a small percentage of the 93,000 people killed in Syria over the last two years.
The Obama administration announced in April that it had "varying degrees of confidence" that sarin had been used in Syria. But they said at the time that they had not been able to determine who was responsible for deploying the gas.
The more conclusive findings announced Thursday were aided by evidence sent to the United States by France, which, along with Britain, announced it had determined that Assad's government had used chemical weapons in the two-year conflict.
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 Congress has been notified of the new U.S. chemical weapons determination, as have international allies. Obama will discuss the assessments, along with broader problems in Syria, next week during the G-8 summit in Northern Ireland.
Obama is also expected to press Russian President Vladimir Putin, Assad's most powerful backers, 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.
The U.S. has so far provided the Syrian rebel army with rations and medical supplies. In April, the administration made a decision in principle to expand its military support to the opposition to include defensive items like night vision goggles, body armor and armored vehicles. That step was announced by Secretary of State John Kerry.
The composition of the defensive military equipment is still being determined in consultation with the opposition military leadership and other nations that are supplying similar material.
Copyright 2013 The Associated Press.