WASHINGTON ― A key Democratic senator is calling on President Joe Biden to seek formal congressional approval for U.S. airstrikes against Houthi-controlled sites in Yemen, signifying growing unease with the administration’s strategy in the region amid fears of broader U.S. entanglement in another Middle Eastern conflict.

“I am eager to hear from the administration about whether they believe there is existing authorization, but I can’t find one,” Sen. Chris Murphy (D-Conn.) told HuffPost on Tuesday. “I think there has to be a war authorization here.”

The U.S. and United Kingdom conducted a new round of strikes in Yemen on Monday, marking the eighth round of attacks by the U.S. military on Iranian-backed Houthi infrastructure in just over 10 days. The strikes hit weapons storage sites and drone systems that the Houthis have been using to attack commercial ships in the Red Sea, in response to Israel’s military campaign on Gaza.

The Biden administration described the initial strikes in Yemen as a “defensive” operation designed to put pressure on the Houthis to end their attacks and to protect global trade. But now, U.S. officials are planning for a “sustained” military campaign targeting the Houthis, according to The Washington Post.

Asked last week if that strategy was working, Biden acknowledged it had not so far.

“Are they stopping the Houthis? No. Are they going to continue? Yes,” Biden told reporters at the White House.

Before Tuesday, the number of lawmakers who have insisted Biden seek congressional approval for the strikes has been fairly small: a contingent of progressives and conservatives who have consistently argued that Congress needs to reassert its constitutional authority in declaring war.

But now, Murphy, a moderate Democrat known for generally backing the White House’s positions, is also pushing for a congressional authorization for military force specifically dealing with Yemen.

“I think there’s reason to give the president some limited permission structure but you can’t fight the Houthis ― an enemy that is named nowhere in existing authorizations ― without legal permission from Congress,” Murphy said.

Sen. Chris Van Hollen (D-Md.), a member of the Senate Foreign Relations Committee, meanwhile, said he feels the question of whether the White House needs Congress to OK the ongoing strikes is “clearly very close to the edge.”

He said that the Houthis appear to be growing in popularity within Yemen as the U.S. steps up its response, and expressed concerns about a broader war in the region.

“There’s a question of: beware of what you dragged into. The Houthis appear so far to relish a fight with the United States,” Van Hollen said.

It’s not just Democrats calling on the Biden administration to give a better explanation for U.S. operations in Yemen, however. GOP Sen. Todd Young (R-Ind.) said the White House should offer “clearer answers” regarding the legality of strikes on the Houthis.

“The American people want us to duly deliberate before committing to some sort of sustained actions, against the Houthis or others, and I think we owe them that,” Young, a former U.S. Marine, said Tuesday.

CORRECTION: A previous version of this story stated incorrectly that Senator Chris Murphy represents Massachusetts. He is a Senator from Connecticut.