WASHINGTON — The Trump administration is developing a new strategy for the war in Syria that would focus more heavily on pushing Iran's military and its proxy forces out of the country, according to five people familiar with the plan.

The new strategy would not involve the U.S. military directly targeting and killing Iranian soldiers or Iran's proxies, however, since that would violate the current U.S. authorization for using force in Syria. The U.S. military does have the right of self-defense under the authorization, and could strike the Iranian military if it felt threatened.

The plan would emphasize political and diplomatic efforts to force Iran out of Syria by squeezing it financially. It would withhold reconstruction aid from areas where Iranian and Russian forces are present, according to three people familiar with the plan. The U.S. would also impose sanctions on Russian and Iranian companies working on reconstruction in Syria.

"There's a real opportunity for the U.S. and its allies to make the Iranian regime pay for its continued occupation of Syria," said Mark Dubowitz, chief executive at the Foundation for Defense of Democracies, a think tank strongly opposed to the Iranian regime.

Driving Iran out of Syria would be one prong in an approach that would also involve continuing to destroy remaining pockets of Islamic State fighters and finding a political transition after the exit of both ISIS and Iran that does not call for Syrian President Bashar al Assad to step aside.

U.S. defense officials worry the increased focus on Iran and the presence of both militaries in Syria could pull the U.S. military closer into conflict.

The U.S. is not allowed to specifically expand the U.S. military mission in Syria to directly target Iranian assets, according to legal experts, because that would put the U.S. on the wrong side of the Authorization for Use of Military Force (AUMF) passed by Congress in 2001. That authorization, which permitted the use of military force against ISIS in Syria, limits U.S. action to targeting groups responsible for the Sept. 11, 2001, attacks and their associates.

"If the new strategy means opening the door to using force against Iran or Iranian military forces in Syria there needs to be a new Authorization for Use of Military Force," said Professor Oona Hathaway of Yale Law School. "Targeting Iran clearly falls outside the scope of the current AUMF which only includes groups with ties to 9/11. Iran doesn't meet that test. It would be amazing if [the Trump administration tries] to make the claim this falls under the current AUMF. That would be stretching this AUMF way past its breaking point."

A defense official said under the new plan the military would continue to talk about the mission in Syria as counter-ISIS, downplaying the fight against Iran, while the White House and State Department would increase their focus on countering Iran by squeezing them economically and diplomatically.

An administration official said that since last year Trump's Syria strategy has pursued four goals — defeating ISIS, deterring Assad's use of chemical weapons, creating a political transition in Damascus, and curbing "Iranian malign influence in Syria so that it cannot threaten the region, to include ensuring the withdrawal of Iranian-backed forces from Syria."

"The United States will continue to seek to hold Assad accountable for his crimes," the official said. "Pursuant to the National Defense Authorization Act for Fiscal Year 2018, the administration will soon submit to Congress a strategy for Syria that reflects the president's key priorities."

'Sometimes it's time to come back home'

In April, President Donald Trump said he wanted to get U.S. troops out of Syria. "I want to bring our troops back home. I want to start rebuilding our nation," he said during a press conference. "We were successful against ISIS … but sometimes it's time to come back home, and we're thinking about that very seriously."

Both U.S. officials and international allies persuaded Trump to keep U.S. troops in Syria by convincing him that leaving was counter to his efforts to get tough on Iran. The president's cabinet, his military leadership, and allies in the Middle East who opposed U.S. withdrawal persuaded him that a U.S. departure would create a vacuum that would be filled by Iran and ISIS. That got Trump's attention, according to the five people familiar with the new plan.

The Trump administration believes its renewed sanctions are having an impact already, and with continued economic pressure Iran will have trouble paying its forces in Syria.

While the new Syria strategy does not include a mandate that Assad must go, it states that the new government cannot have close ties to Iran and must be willing to prosecute individuals who have committed crimes against humanity, according to the five people familiar with the plan.

The U.S.-backed Syrian opposition forces have eliminated most of the ISIS presence in Syria, taking back more than 98 percent of the land the terror group once held there. But U.S. military officials warn that small pockets of ISIS fighters will continue to pop up as the fight there becomes an insurgency.

And even though Trump has spoken about bringing all U.S. troops home, experts believe this new strategy buys the military more time.

"His instincts are to get out," Robert Malley, who was a senior adviser to President Barack Obama on the Middle East and counter-ISIS campaign, said of Trump. "They found a way at least for now to keep him in."

"How long it lasts? Who knows?"