Armed men in uniform identified by allied Syrian fighters as U.S. Special Operations forces ride in the back of a pickup truck in the village of Fatisah in the northern Syrian province of Raqqa on May 25, 2016. (Delil Souleiman/AFP/Getty Images)

A Pentagon plan for the coming assault on Raqqa, the Islamic State capital in Syria, calls for significant U.S. military participation, including increased Special Operations forces, attack helicopters and artillery, and arms supplies to the main Syrian Kurdish and Arab fighting force on the ground, according to U.S. officials.

The military’s favored option among several variations currently under White House review, the proposal would ease a number of restrictions on U.S. activities imposed during the Obama administration.

Officials involved in the planning have proposed lifting a cap on the size of the U.S. military contingent in Syria, currently numbering about 500 Special Operations trainers and advisers to the combined Syrian Democratic Forces, or SDF. While the Americans would not be directly involved in ground combat, the proposal would allow them to work closer to the front line and would delegate more decision-making authority down the military line from Washington.

President Trump, who campaigned on a pledge to expand the fight against the militants in Syria, Iraq and beyond, received the plan Monday after giving the Pentagon 30 days to prepare it.

But in a conflict where nothing has been as simple as anticipated, the Raqqa offensive has already sparked new alliances. In just the past two days, U.S. forces intended for the Raqqa battle have had to detour to a town in northern Syria to head off a confrontation between two American allied forces — Turkish and Syrian Kurdish fighters. There, they have found themselves effectively side by side with Russian and Syrian government forces with the same apparent objective.

Approval of the Raqqa plan would effectively shut the door on Turkey’s demands that Syrian Kurds, considered terrorists by Ankara, be denied U.S. equipment and kept out of the upcoming offensive. Turkish President Recep Tayyip Erdogan has said that arming and including the Kurdish People’s Protection Units, or YPG, in the operation is unacceptable and has vowed to move his own troops and Turkish-allied Syrian rebel forces toward Raqqa.

U.S. officials, some of whom spoke on the condition of anonymity about the still-secret planning, believe Erdogan’s tough talk is motivated primarily by domestic politics, specifically a desire to bolster prospects for an April 16 nationwide referendum that would transform Turkey’s governing system to give more power to the presidency.

Lt. Gen. Stephen Townsend, the Baghdad-based U.S. commander of the anti-Islamic State coalition, told reporters Wednesday that there was “zero evidence” that the YPG was a threat to Turkey. With some apparent exasperation, Townsend called on all anti-Islamic State forces in northern Syria to stop fighting among themselves and concentrate on the best way to beat the militants.

U.S. talks with Turkey, a NATO ally and coalition member, are ongoing. But events over the past several days in and around the town of Manbij have injected a new element in the conflict that could either help the Americans avoid a direct clash with Ankara, or set the many forces now converging on the town on the path toward a new confrontation.

Manbij, located near the Turkish border about 85 miles northwest of Raqqa, was captured by the Islamic State three years ago and retaken last August by the YPG, backed by U.S. airstrikes and advisers. The town now forms the western edge of a militant-cleared border strip extending to neighboring Iraq.

The United States had promised the Turks that Kurdish control would not extend to the west beyond the nearby Euphrates River, and Manbij was turned over to the Manbij Military Council, Arab fighters within the SDF. Kurdish police are in charge of local security, but the Americans have insisted that YPG fighters have largely left the scene.

Turkey disagrees and has long threatened to forcibly eject the Kurds, who it says are affiliated with the Kurdistan Workers’ Party, or PKK, a designated terrorist organization in both Turkey and the United States that is waging an insurgency inside Turkey for greater autonomy. After Turkish troops and their Syrian rebel allies took the nearby Syrian town of Al-Bab from the Islamic State on Feb. 23, the Turkish-led force began advancing toward Manbij and has captured at least two villages.

On Thursday, as Turkish shells reached the outskirts of the town, the Manbij Military Council announced it had invited the government of Syrian President Bashar al-Assad to take over several nearby villages as part of a deal brokered by Russia to avoid conflict with the Turks.

On Friday, Moscow announced that Russian and Syrian “humanitarian” convoys were heading toward Manbij. Pentagon spokesman Capt. Jeff Davis told reporters in Washington that the convoys also included “some armored equipment.”

Davis said that the U.S. government had been “informed” of the movements by Russia but that “it’s nothing that we’re party to.”

Meanwhile, photographs posted on social media showed U.S. military vehicles headed into Manbij from the east.

On Saturday, the U.S. military confirmed that it had “increased force presence in and around Manbij to deter hostile acts, enhance governance and ensure there’s no persistent YPG presence,” effectively inserting U.S. forces to keep two coalition members — Turkey and the Syrian Kurds — from fighting.

In postings on his Twitter account, coalition spokesman Col. John L. Dorrian said the coalition “has taken this deliberate action to reassure Coalition [members] & partner forces, deter aggression and keep focus on defeating ISIS,” an acronym for the Islamic State.

The United States and Russia have managed to avoid confrontation in Syria’s separate civil war, where they are on opposing sides. Trump has said repeatedly that the two powers should cooperate against the Islamic State, and he has indicated that the future of Russia-backed Assad is of less concern to him.

The Pentagon disapproves of possible U.S.-Russia cooperation, although U.S. officials are not unhappy at the buffer Russia and Syria now appear to be creating between Turkey and the Kurds, or the prospect of the Syrian government moving into Manbij. A positive result, officials said, would not only prevent Turkish forces and their Syrian allies — many of whom are on the jihadist side of the anti-Assad rebel coalition — from moving into the town, but it would also potentially push any remaining YPG forces to the eastern side of the Euphrates.

While Turkey has supported rebel forces fighting against Assad, it has never come into direct conflict with the Syrian military, and U.S. officials believe it would far rather have the Syrian government in charge of Manbij than the Kurds. There are hopes that Moscow, which has been simultaneously working to improve relations with Turkey, can help persuade Erdogan to back off.

What the Americans manifestly do not want to see happen is the creation of a new military front and potential conflagration around Manbij that would drain both attention and resources away from plans for Raqqa. With the city believed to be the center of Islamic State planning for overseas attacks, the offensive is seen as urgent and has already been delayed from original plans to begin in February.

In his final days in office, former president Barack Obama approved plans to send two or three Apache attack helicopters to the Syrian theater but deferred approval of arming the Kurds as part of the SDF. Rather than moving immediately on the plan already in place, Trump at the end of January ordered the Pentagon to draw up new options by the end of February.

With the only real alternative being to use U.S. ground troops against Raqqa, Defense Secretary Jim Mattis has stuck with the basic outline of the plan drawn up under Obama, officials said. The combined Syrian Arab-Kurdish force, now numbering more than 50,000, has moved steadily to within less than six miles of the outskirts of Raqqa in an isolation phase that is expected to be completed in the coming weeks.

Even if Turkey does direct its forces south toward Raqqa, the hope is that the difficult terrain they would have to travel would prevent them from reaching there until after the offensive is well underway.

Rather than a wholesale revision, the new proposal calls for increased U.S. participation, with more personnel and equipment and less-restrictive rules. As they have in support of the Iraqi military in Mosul, U.S. fixed-wing aircraft and attack helicopters would actively back the ground force. U.S. owned and operated artillery would be moved into Syria to pound the militants from afar, while more Special Operations troops would move closer to the front lines — requiring more U.S. military assets to protect them.

The SDF — both Kurds and Arabs — would be supplied with weaponry along with vehicles and equipment to travel through and disarm what are expected to be extensive minefields and other improvised explosive devices along the way.

Trump’s executive order also directed the Pentagon to recommend changes to Obama administration restrictions on military rules of engagement that went beyond those required by international law. Principal among them is an Obama executive order, signed last summer, imposing strict rules to avoid civilian casualties. It is not known whether the new military proposal would lift those restrictions.

Sly reported from Beirut. Zakaria Zakaria in Istanbul and Heba Habib in Stockholm contributed to this report.