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The general said that establishing a no-fly zone over Syria, as some lawmakers have called for, would dent the regime’s air power but would not be a decisive blow leading to Mr. Assad’s ouster and instead would draw the U.S. deeper into the conflict.

Lawmakers on Capitol Hill said some sort of intervention is needed to stop the civil war, which has raged since 2011 and has killed more than 100,0000, according to U.N. estimates.

“The U.S. has two options: continue to largely stand on the sidelines as the regime slaughters its own people, or tip the balance of power against a brutal dictator by degrading its ability to attack civilians,” Mr. Engel said in a statement. “If we are to salvage what remains of our credibility in the region, we must act soon.”

Sen. Robert P. Casey Jr., Pennsylvania Democrat, said the latest chemical weapons revelations mean that Mr. Obama’s red line has been crossed again.

“Our national security interests continue to be at stake — every day that Assad remains in power is a benefit to Iran and the terrorist group Hezbollah,” he said.

The White House indicated that if chemical weapons were used, that could help solidify a coalition opposed to the Syrian regime, which still has support from longtime allies including Russia.

The Russian Foreign Ministry suggested that the attack may have been a “provocation” by the rebels, according to a report by RT, a network that broadcasts Russian views.

U.S. officials said earlier this year they had officially concluded that chemical weapons have been used in a few attacks and were reasonably sure the culprits were Syrian government forces. But given past questions about U.S. accusations of weapons of mass destruction, particularly in Iraq, the administration has been tentative in making more definitive claims.

That leaves the investigation to the U.N. team that arrived in Syria on Sunday.

The team negotiated with the Syrian government to gain access to the sites, but now will have to renegotiate if the investigators are to gain access to locations of this week’s attack.