BEIRUT — With the Trump administration’s decision to supply Syria’s Kurdish fighters with heavier weapons, U.S. troops inside Syria are in the crossfire between Turkey, a powerful NATO ally, and the Kurdish fighters that Ankara deems as terrorists.

In only a few months under President Donald Trump, the U.S. has almost doubled the number of troops in northern Syria, taking a highly visible role that also risks a backlash from militants such as the Islamic State group and the Iranian-backed Hezbollah, and even pro-Turkey Syrian fighters angered by the U.S. move to arm the Kurds.

Tuesday’s decision to arm the Kurds is a public rebuff to Turkey’s President Recep Tayyip Erdogan a week before he meets Trump in Washington. For months, Erdogan has been trying to convince the U.S. to cut off its support for the Kurds and partner instead with Turkey-backed fighters to liberate the Islamic State group stronghold of Raqqa.

The dispute could ignite more fighting between Turkey and the Kurds as they gear up for a major operation to retake the city, with U.S. troops smack in the middle.

The growing U.S. involvement in Syria’s civil war stands in sharp contrast to the caution adopted by former President Barack Obama and has alarmed officials in Damascus and its backers in Tehran.

Last month, Trump gave orders to fire 59 Tomahawk missiles at an air base in central Syria in response to a devastating chemical weapons attack blamed on Syrian President Bashar Assad’s forces. It was the first time the U.S. has attacked Syrian forces in the six-year civil war.

Although U.S. officials have said repeatedly since then that the war against the Islamic State group in Syria remains the priority, that strike — coupled with the buildup of forces in the north — has raised speculation of longer term U.S. ambitions in Syria and concerns about a more permanent project.

Under Trump, the Pentagon has made quiet, incremental additions to troop levels in Syria, adding hundreds of Marines to provide artillery support and sending more advisers to work with Kurdish units ahead of the fight for Raqqa. The official limit on U.S. troops has remained at 503 since shortly before Obama left office, but U.S. commanders this year have added hundreds of troops, including a Marine artillery unit, on what they call a temporary basis, raising the total to about 900.

These have taken on a more visible role, often aimed at keeping Turkey and the Kurds from battling each other and focused instead on the fight against IS.

On April 30, U.S. forces accompanied by Kurdish People’s Protection Units (YPG) began patrols along the Turkey-Syria border, acting as a buffer after Turkish airstrikes in the area killed 20 Kurdish fighters. The spokesman for the U.S.-led coalition against IS, Col. John Dorrian, said U.S. troops were about 6 miles from the strikes and put American forces at risk.

Images of U.S. troops in armored personnel carriers with American flags and maneuvering down rural roads in northern Syria spread quickly on social media, triggering alarm in a region where there are political sensitivities about the footprint of U.S. troops and fears about occupation forces.