American military commanders have long argued that arming the People’s Protection Units, or Y.P.G., a Kurdish militia fighting alongside Syrian Arab forces against the Islamic State, is the fastest way to seize Raqqa.
But Turkey has strongly objected to such a move, raising fears of a backlash that could prompt the Turks to curtail their cooperation with Washington in the struggle against the Islamic State, also known as ISIS or ISIL. Turkish officials have issued veiled threats that they would shut down allied operations at Incirlik Air Base, the major air hub for American and allied warplanes in the battle.
Turkey’s National Security Council said on Wednesday that the Trump administration’s decision to arm the Kurdish militia in Syria was “not befitting of an alliance.”
Equipment provided to the Kurds, which is being drawn from stockpiles in the region, will be limited in quantity and by mission, and will be doled out incrementally as objectives are reached, Colonel Dillon said.
American military officials have insisted for months that the weapons are needed to help the lightly armed Kurdish and Arab fighters cope with urban warfare in Raqqa against Islamic State militants who have been building fortifications for months and are equipped with car bombs and even some tanks they captured from the Syrian Army.
Thousands of Syrian Kurdish and Arab fighters have pushed to within about two miles of the city, where American military officials and humanitarian groups are bracing for a bloody, monthslong battle — similar to the fight Iraqi forces have carried out in Mosul, another Islamic State stronghold. In preparation for the assault, American and allied warplanes have intensified airstrikes against militant forces in and around Raqqa in recent weeks.
At the same time, the Kurdish and Arab militias, which American Special Operations forces are advising, have been tightening a rough cordon around most of the city, capturing dozens of small towns and villages as they go. The fighters have surrounded Raqqa from the north, the west and the east. The extremists still have an exit from the south, even though the American-led coalition destroyed two southern bridges over the Euphrates River.
To address Turkish concerns that the arms might be used against them after the fight for Raqqa is over, the supply of weapons and ammunition will be limited to what the Kurds and Arab fighters need to carry out specific operations, American officials said.
“Wherever possible, our advisers will monitor the use of the weapons and supplies we give the Kurdish elements of the S.D.F., ensuring use only against ISIS,” Colonel Dillon said. “Any alleged misuse or diversion of U.S. support will be taken seriously and lead to the possible curtailment of support, if verified.”
The United States has long worked with the Y.P.G. under the umbrella of the Syrian Democratic Forces. The American military has always emphasized that those forces include Arab fighters, who make up nearly half of the total force and most of the fighters near Raqqa. But the Y.P.G. is generally considered to have the most experienced and battle-hardened fighters.
The Turkish government has long insisted that the Kurdish militia is closely linked to the Kurdistan Workers’ Party, a separatist group. That group is listed by Turkey, the United States and Europe as a terrorist organization.
Some Syria analysts said on Wednesday that the militias would need to include more of the local Sunni Arab tribes to maintain the fighting force’s potency after the battle for Raqqa, if they aim to vanquish pockets of remaining Islamic State resistance in the region.
“Arming the Kurdish elements of the S.D.F. will make them more militarily effective against ISIS in Raqqa,” said Andrew J. Tabler, an expert on Syria at the Washington Institute for Near East Policy. But, he added, referring to President Bashar al-Assad of Syria, “if they don’t expand to include more of the Sunni Arab tribes of the Euphrates River valley, who make up the majority there, the S.D.F. will have a hard time holding that area because of the Kurdish-Arab split, leaving that area vulnerable for an Assad regime comeback.”