White House spokesman Josh Earnest said the administration wants a U.N. investigation to determine if chemical weapons were used. U.N. investigators already are in Syria, looking into alleged attacks earlier this year.
Syrian rebels alleged hundreds of people had been killed in a chemical attack by the government near Damascus. President Obama has said the United States would become involved if the use of chemical weapons was proved.
"If the Syrian government has nothing to hide and is truly committed to an impartial and credible investigation of chemical weapons use in Syria, it will facilitate the U.N. team's immediate and unfettered access to this site," he said.
The news came as Joint Chiefs of Staff Chairman Gen. Martin Dempsey said the United States warned rebel groups in Syria are not aligned with U.S. interests.
In a letter this week to Rep. Eliot Engle, D-N.Y., the ranking minority member on the House Foreign Affairs Committee, Dempsey said the United States must seek out and support whatever group is likely "to promote their interests and ours when the balance shifts in their favor. Today, they are not," The Hill reported.
"Syria today is not about choosing between two sides but rather about choosing one among many sides," Dempsey wrote in arguing against U.S. intervention.
Many U.S. leaders from both parties have called on Obama to set up a no-fly zone. Dempsey in his letter said the military is "pragmatic about the limits of military force."
The Syrian rebels include a broad range of groups from Islamists with ties to al-Qaida to secular opponents of President Bashar Assad.
"I reject the notion that our involvement in Syria would simply constitute 'choosing sides' between one armed group and another," Engel said. "Rather, our involvement represents a choice between hastening the end of the Assad regime or continuing to allow the cycle of violence, displacement and terror to continue unabated."