Trump, who charged former president Barack Obama with being weak on Syria, gave the Pentagon 30 days to prepare a new plan to counter the Islamic State, and Mattis submitted a broad outline to the White House at the end of February. Gen. Joseph Votel, head of U.S. Central Command, has been filling in more details for that outline, including by how much to increase the U.S. ground presence in Syria. Votel is set to forward his recommendations to Mattis by the end of the month, and the Pentagon secretary is likely to sign off on them, according to a defense official familiar with the deliberations.

While the new contingent of U.S. troops would initially not play a combat role, they would be entering an increasingly complex and dangerous battlefield. In recent weeks, U.S. Army Rangers have been sent to the city of Manbij west of Raqqa to deter Russian, Turkish and Syrian opposition forces all operating in the area, while a Marine artillery battery recently deployed near Raqqa has already come under fire, according to a defense official with direct knowledge of their operations.

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The moves would also mark a departure from the Obama administration, which resisted committing more ground troops to Syria.

The implementation of the proposed plan, however, relies on a number of variables that have yet to be determined, including how much to arm Kurdish and Arab troops on the ground, and what part regional actors, such as Turkey, might have in the Raqqa campaign.

The new troops, if sent, would be focused on supporting Kurdish and Arab fighters in northern Syria battling the Islamic State. Under the plan, the added American forces would act primarily as advisers, offering expertise on bomb disposal and coordinating air support for the coalition of Kurds and Arabs, also known as the Syrian Democratic Forces.

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About 500 U.S. Special Operations forces are already in Syria operating alongside the SDF, in addition to about 250 Rangers and 200 Marines. The new U.S. troops, if approved, would probably come from parts of both the 24th Marine Expeditionary Unit — a flotilla of ships loaded with 2,200 Marines that is now steaming toward the region — and the U.S. Army’s 82nd Airborne Division, from which 2,500 troops are headed to Kuwait. These conventional troops would supplement the Special Operations forces already on the ground and operate much like their counterparts fighting in the Iraqi city of Mosul.

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“This would still be by, with and through our local partners on the ground,” one defense official said of the potential surge. The official, like others, spoke on the condition of anonymity to discuss plans that have not yet been made public.

The new Syria deployments are set to occur in tandem with a likely White House decision that would officially abolish the troop caps that were put in place for U.S. forces in Iraq and Syria by the Obama administration. The number of troops in Iraq and Syria were officially capped by the previous administration at about 5,000 and 500, respectively. Military commanders have said in the past that the caps have split up units for the sake of keeping troop numbers low.

“If the caps were removed, it would allow us to fight as we train,” said the defense official who discussed the potential surge. “Military doctrine promotes agility, and it would help us respond as conditions dictate.”

White House officials have said they expect the Raqqa campaign to move forward much faster than the current offensive to retake Mosul and have insisted on the need for other options if the current plan bogs down.

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“The lesson learned from Libya and Iraq is that you better have a Plan B force or you are asking for trouble,” said a senior U.S. official with knowledge of the planning.

One option would be to embed U.S. forces alongside Syrian Kurdish and Arab forces, potentially pushing U.S. soldiers and Marines into a direct combat role. Those American forces could also be supplemented by troops from the United Arab Emirates, Saudi Arabia and Jordan.

“Basically you would have a force bolstered by allies with U.S. leadership,” the senior U.S. official said.

Trump made clear throughout the campaign and since his inauguration that he was willing to increase the number of U.S. troops in Syria. At the same time, however, he said he expected regional neighbors who shared U.S. goals in Syria to carry the primary load there.

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“This is definitely a new administration,” said Andrew Exum, a former deputy assistant secretary of defense for Middle East policy. While the Trump administration isn’t breaking from the overall strategy of relying on partner forces, its decision to get rid of force caps and commit conventional troops in Syria is “certainly a change,” Exum said.

Trump has spoken favorably about cooperating more with Russia in Syria on counterterrorism operations, although there is little indication that will soon become a reality. Russia’s air and ground forces are fighting a civil war on behalf of Syrian President Bashar al-Assad. The United States and NATO-ally Turkey back separate rebel forces in both the fight against Assad and the anti-Islamic State campaign.

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Partners including Saudi Arabia have said for years that they are willing to send troops to Syria, as long as the United States showed it was willing to put comparable skin in the game. The extent to which the Saudis have been included in U.S. planning remains unclear. On Tuesday, Trump met at the White House with Mohammed bin Salman, the Saudi deputy crown prince and defense minister. The Saudi foreign minister and his counterpart from the United Arab Emirates are also in Washington this week and met Monday with Secretary of State Rex Tillerson.

Turkey has said it will send ground forces to help seize Raqqa, but that option is complicated by Turkish insistence that its participation depends on the United States severing ties with Syrian Kurdish fighters, called the People’s Protection Units, or YPG. While the Pentagon considers the YPG to be the most effective local Syrian fighting force, Turkey has labeled it a terrorist group.

Greg Jaffe, Karen DeYoung and Missy Ryan contributed to this report.