Dem. Rep.: Britain does not 'have veto power over what we do'

Pressed if he would support unilateral action, the top Democrat said he "would much prefer, obviously, if there was an international coalition."

But, Engel said, "I think if we have to, then I think we will."

The White House said on Thursday that the U.K. vote would not deter its plans.

"As we’ve said, President Obama’s decision-making will be guided by what is in the best interests of the United States," said National Security Council spokeswoman Caitlin Hayden. "He believes that there are core interests at stake for the United States and that countries who violate international norms regarding chemical weapons need to be held accountable."

Engel, who was among the lawmakers briefed on a evening conference call with National Security Adviser Susan Rice, Secretary of State John Kerry and Secretary of Defense Chuck Hagel, also said he was convinced there was "no doubt that [Syrian President] Bashar al-Assad was directly involved" in the chemical weapons attack.

In recent days, unnamed U.S. officials have told The Associated Press and Foreign Policy that intelligence assessments were unable to prove definitively that Assad directly authorized the attack, although the White House has brushed off those reports.

“Nothing happens in Syria without his involvement," Engel said. "It’s not where some underling can make a decision and do it. He has to approve this. And he is aligning himself with a terrorist group, Hezbollah. He’s become Iran’s proxy in Syria.”