WASHINGTON — President Barack Obama faced pushback Thursday on a potential military strike against Syria, with wary lawmakers in both the U.S. and Britain demanding more proof that Bashar Assad’s government was responsible for a suspected chemical weapons attack.

Even so, military action could come within days.

Top U.S. officials scheduled a briefing with lawmakers late Thursday to present a case that Syrian forces killed hundreds of civilians with chemicals last week. But key evidence is classified, they said, suggesting the public won’t see the most convincing material tying Assad’s government to the attack.

New hurdles appeared to slow the formation of an international coalition to move against Syria or to sanction whatever the U.S. might do on its own.

British Prime Minister David Cameron said his country would hold off on joining any military efforts until a U.N. chemical weapons inspection team releases its findings, probably no earlier than the weekend. At the United Nations, Russia blocked British efforts to seek a resolution authorizing the use of force.

But it appeared unlikely that any balking among allies, flak thrown up by Russia or skepticism in Congress would slow Obama’s hand much longer.

He said late Wednesday that while he had not settled on a response, the U.S. has concluded that Assad’s government perpetrated a chemical weapons attack, “and if that’s so, then there need to be international consequences.”

The administration planned an intelligence teleconference briefing Thursday evening on Syria for leaders of the House and Senate and the national security committees in Congress, U.S. officials and congressional aides said.

Earlier, Obama spoke by phone with Republican House Speaker John Boehner, who has asked the president to make a sharper case on the legal justification for any military strike and its objective.

Many Congress members were pressing Obama to explain the need for military action and address fears that such a move might draw the U.S. deeper into the Syrian civil war. Both Democrats and Republicans were among lawmakers protesting that Obama hasn’t made the case for a military strike, with some arguing that the president needs congressional authorization to order an attack.