By Kristina Peterson 

WASHINGTON -- Momentum is growing on Capitol Hill for new sanctions against supporters of Syrian President Bashar al-Assad's regime, as many lawmakers' patience with the Obama administration's approach wears thin following the collapse of the latest cease-fire agreement.

President Barack Obama has opposed stepping up sanctions. House Speaker Paul Ryan (R., Wis.) urged the White House last week to lift behind-the-scenes pressure that has prevented House Democrats from backing sanctions legislation headed to the chamber's floor.

"At no point has the Assad regime stopped committing atrocities against the Syrian people," Mr. Ryan told reporters Thursday. "This administration is essentially protecting some of the world's worst war criminals. That is why I am calling on the administration to allow House Democrats to work with us to pass these sanctions in short order."

White House press secretary Josh Earnest said Mr. Obama opposes the Syria sanctions bill because he believes applying unilateral sanctions isn't as effective as coordinating sanctions with U.S. allies.

"We've already got the authority that we need to impose sanctions against the Assad regime if we believe it's going to advance our interests in that part of the world," Mr. Earnest said Friday. "Unilateral sanctions imposed by the United States Congress are not likely to have the desired effect."

It isn't clear when either chamber might take action, although a furious Syrian and Russian bombing campaign in Aleppo is drawing world-wide condemnation. Both congressional chambers are expected to leave Washington at the end of the week until after the November election.

Republican leaders hoped to pass the sanctions bill last week under expedited procedures that require the support of two-thirds of voting lawmakers. But the White House pressed House Democrats to withhold their support for the bill to protect the cease-fire in Syria brokered this month by the U.S. and Russia, a Damascus ally, and allow for diplomatic talks at the annual United Nations General Assembly meeting last week, according to House aides from both parties.

The cease-fire broke down after a humanitarian aid convoy was attacked in northern Syria on Monday, killing 12 people. U.S. officials said Russia was ultimately responsible for the strike.

The U.S. currently has longstanding sanctions that prohibit Americans from conducting financial or commercial dealings with Syria. The new legislation by House Foreign Affairs Committee Chairman Ed Royce (R., Calif.) and Rep. Eliot Engel of New York, the panel's top Democrat, is designed to increase pressure on the military supply chain in Syria by sanctioning Syria's energy industry and penalizing anyone doing business with its telecommunication or transportation sectors, a Democratic congressional aide said. Sanctions on those providing aircraft or spare parts to Syria's airlines are designed to ground the Syrian air force, the aide said.

"The legislation significantly expands the scope and coercive impact of the existing secondary sanctions," said Mark Dubowitz, executive director of the Foundation for Defense of Democracies.

The bill would prescribe mandatory sanctions the administration must impose, and would levy them across entire sectors rather than against individuals, making them harder to evade via techniques such as front companies, Mr. Dubowitz said.

Under the bill, the president would have the ability to waive sanctions on a case-by-case basis.

"While we need to give the cease-fire every chance to succeed, we also need to be honest that this effort is a long shot," Mr. Engel said in a statement Friday. "We need to use every tool at our disposal to get Russia to commit to a meaningful and effective cessation of hostilities."

The legislation also would require the White House to submit reports to Congress on human-rights abusers and on the potential effectiveness and risks of establishing and maintaining a no-fly zone or a safe zone in Syria.

Rep. Gerald Connolly (D., Va.) said he saw no need to hold back from passing sanctions against Mr. Assad's backers.

"We need to continue to find ways to dislodge him from power," said Mr. Connolly, who said the White House hadn't contacted him about the bill. "I don't believe there can ever be a solution to restoring Syrian unity, if that's even a realistic goal, so long as Assad is in power."

Lawmakers from both parties in the Senate have expressed increasing alarm about the state of the Syrian conflict.

"We will be watching for progress in the House and would be willing to consider legislation that could help alleviate the devastating humanitarian crisis caused by the Assad regime in Syria," Senate Foreign Relations Committee Chairman Bob Corker (R., Tenn.) said in a statement Friday.

The Senate panel's top Democrat, Sen. Ben Cardin of Maryland, told reporters last week that there was growing support for looking into some type of "enforced area in Syria in which people are safe."

--Carol E. Lee contributed to this article.

Write to Kristina Peterson at kristina.peterson@wsj.com

(END) Dow Jones Newswires

September 25, 2016 15:50 ET (19:50 GMT)