A member of the YPG uses a radio in a village close to the frontline in Tal Samin, Syria, in November, during the operation to isolate Raqqa. (Alice Martins/For The Washington Post)

President Trump’s developing plan to defeat the Islamic State may lead to significant alterations in the Syria strategy that Trump inherited from Barack Obama, including a reduction or elimination of both long-standing U.S. support for moderate opposition forces fighting against the Syrian government and the use of Syrian Kurdish fighters as the main U.S. proxy force against the militants, according to U.S. officials.

A memorandum signed late last month by Trump ordered the Pentagon and other national security agencies to draft a new proposal by late February. Trump has made clear in public statements both before and since his inauguration that he is eager to increase U.S. firepower against the militants, and willing to add more troops beyond about 500 U.S. Special Operations troops currently on the ground in Syria.

In addition to calling for “new coalition partners,” possibly to include operational coordination with Russia, Trump also ordered recommendations to change any existing military rules of engagement that are more stringent than what is required by international law.

The most prominent of these are a series of restrictions, contained in an executive order Obama signed last summer, designed to limit the number of civilian casualties caused by U.S. air attacks.

Senior officials familiar with the effort said the overall goal is to narrow the U.S. lens to focus more intensely on the Islamic State and other terrorist groups, without the distractions of Syria’s civil war, the needs of “nation-building,” or the promotion of democracy.

“We’re still sorting it out,” ­Defense Secretary Jim Mattis said of the Syria planning during a Monday visit to Baghdad. “It’s not been all decided on who’s going to do what and where. We’re working together to sort it out.”

On the table when Obama left office was an urgent Pentagon request for approval of its existing plan to conquer Raqqa, the Islamic State’s de facto capital in Syria. The plan called for arming the Syrian Kurds of the People’s Protection Units, or YPG, whose fighters have been the main force in U.S.-assisted efforts to clear the Islamic State from northern Syria, along the Turkish border.

Backed by U.S. and coalition air power and Special Operations advisers behind the front lines, the YPG and a smaller group of ­Syrian Arab fighters are now within a few dozen miles of Raqqa, where they are awaiting a final White House decision.

The Obama approach has been vehemently opposed by Turkey, which considers the YPG an extension of its own separatist and outlawed Kurdistan Workers’ Party, or PKK. U.S. cooperation with the Syrian Kurds caused a significant breach between the Obama administration and the government of Turkish President Recep Tayyip Erdogan, and there have been high hopes in Ankara that the Trump White House would be more receptive to its concerns.

In a Feb. 7 phone call with Erdogan, Trump was noncommittal. Since then, top U.S. national security officials — including CIA Director Mike Pompeo, Mattis and the Joint Chiefs of Staff chairman, Gen. Joseph F. Dunford Jr. — have been told in face-to-face talks with their Turkish counterparts how important the issue is to Ankara, according to those Turkish officials.

In a meeting in Munich on Monday, Vice President Pence reportedly told Turkish Prime Minister Binali Yildirim that the United States wants a “fresh start” in relations with Turkey. Yildirim told Turkish reporters he was optimistic that a new approach will emerge for Raqqa.

Turkey has proposed several different versions of a plan to take the city, all of which involve Turkish troops, more U.S. personnel and Syrian Arab fighters, and exclude the YPG. It has also pushed for establishing a safe zone in a 25-mile deep strip along the border — including territory in which the YPG is already operating with U.S. acquiescence.

The proposals are politically attractive to the Trump administration, which has issued its own call for a safe zone in which displaced Syrians could be lodged. They also potentially open the door to cooperation with Russia, whose warplanes have aided Turkish troops that have already moved into Syrian territory to the town of al-Bab, west of the area of Kurdish control.

Turkey could also provide artillery and transport considered crucial for the fight against the Islamic State in Raqqa. In the operation to eject the militants from the city of Mosul, in next-door Iraq, conventional Iraqi army ground forces have made good use of heavy weaponry, but no such equipment is available in Syria.

Gen. Joseph Votel, head of the U.S. Central Command, referred to the problem during a visit Wednesday to Amman, Jordan, telling reporters traveling with him that the Raqqa offensive might require more U.S. troops. Local forces in Syria “don’t have as good mobility. They don’t have as much firepower,” he said, apparently referring to the Kurdish-dominated force, “so we have to be prepared to fill in some of those gaps for them.”

But Turkey’s plans also pose significant challenges. In addition to assuming a larger U.S. force on the ground, they require the U.S. military to abandon a trusted force that has played the leading role in driving the Islamic State from much of northern ­Syria, in exchange for Turkish and Syrian Arab forces about which there are significant reservations. A midstream change of that magnitude could also mean a significant delay in a Raqqa offensive, initially anticipated to take place this spring.

Both the U.S. military and the intelligence community have doubts about the wisdom of cooperating with Russia, whose primary interest is preserving its own strategic equities in Syria and whose principal ally there is Iran.

As it considers Turkey’s proposals, the Pentagon is maintaining a constant dialogue with Ankara to ensure that there is a feasible way forward to secure Raqqa, said a senior Defense official, one of several who discussed planning and were not authorized to comment publicly.

Also to be decided is the future of a three-year-old CIA program to train and equip the moderate opposition fighting against the government of Syrian President Bashar al-Assad and his Russian and Iranian allies. Its scope has been limited by unease over sending sophisticated weaponry to groups that fight in proximity to an al-Qaeda affiliate also attacking the government.

During his presidential campaign, Trump regularly disparaged the opposition fighters, saying, “We have no idea who these people are.” Instead, he suggested that the United States should shift its attention away from ­Syria’s civil war and join forces with Assad and Russia against the Islamic State.

Since then, Russian and Syrian bombing — bolstered by Iran, Lebanon’s Hezbollah, and Shiite militia forces from Iraq — has driven thousands of opposition fighters from Aleppo and other parts of Syria into rural enclaves and into northwest Syria’s Idlib province, an area dominated by Jabhat al-Nusra, the al-Qaeda group now known as Jabhat Fatah al-Sham.

In recent weeks, opposition groups have complained that their supply lines appear to be frozen, although it is unclear if that is a temporary state or the administration has already made a more permanent decision to cut them off.

Michael Birnbaum in Brussels and Dan Lamothe and Thomas Gibbons-Neff in Washington contributed to this report.