The White House on Wednesday condemned reports that Syrian President Bashar al-Assad’s regime used chemical weapons against anti-government rebels, a clash that opposition forces claim killed at least 100 people.

“The United States is deeply concerned by reports that hundreds of Syrian civilians have been killed in an attack by Syrian government forces, including by the use of chemical weapons, near Damascus earlier today,” White House Deputy Press Secretary Josh Earnest said. “We are working urgently to gather additional information.”

The timing of the violence was surprising in that investigators from the United Nations were in Syria to investigate sites where Assad’s regime is accused of using chemical weapons. The White House called on the Syrian government to immediately allow the U.N. investigation to proceed.

“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,” Earnest said.

The Syrian government dismissed the rebels’ claims as baseless, accusing the opposition of trying to drum up public support during the U.N. investigation.

Obama a year ago declared that if Syria unleashed chemical weapons against rebels, such action would cross his “red line” for intervention there. However, the administration has cautiously sidestepped calls for a heightened response to the violence, aside from sending small arms and ammunition to the anti-government rebels.

Administration officials have privately expressed doubts about whether the rebel forces share U.S. interests in the volatile region.

The death count in Syria Wednesday ranged from 100 to 1,300. If confirmed, it would mark the deadliest day in a civil war that has claimed tens of thousands of lives.

The White House Wednesday said it did not have independent confirmation of chemical weapons use. And administration officials declined to outline repercussions if Assad blocked the U.N. investigation.