WASHINGTON — The United States started Tuesday to deliver weapons to Kurdish fighters closing in on the Islamic State stronghold of Raqqa, Syria, the Pentagon said.

Spokesman Eric Pahon said Tuesday’s weapons delivery to the Syrian Kurds included small arms and ammunition. It marks the beginning of a campaign to better equip Kurdish allies that the U.S.-led coalition believes are the best fighting force against the Islamic State, even though arming them has infuriated NATO ally Turkey.

Turkey considers the Kurdish fighters to be terrorists. The U.S. has promised to mete out the equipment incrementally, based on the mission, to insure weapons aren’t used by Kurdish groups in Turkey.

On Tuesday, Kurdish-led fighters in Syria closed within about 2 miles (three kilometers) of Raqqa, where they expect to face a long and deadly battle. Roadside bombs and other explosive devices are believed to be planted along their routes and inside the city.

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U.S. officials have said the weapons deliveries will include heavy machine guns, ammunition, 120mm mortars, armored vehicles and possibly TOW anti-tank missiles. They said the U.S. would not provide artillery or surface-to-air missiles.

Separately Tuesday, the Pentagon ratcheted up threats against pro-Syrian government forces patrolling an area near the Jordanian border where the U.S.-led coalition is training allied rebels. Officials described the pro-government forces as Iranian-backed.

The U.S. dropped leaflets warning the forces to leave the area and American military officials said the same message was conveyed in recent calls with Russian commanders. Navy Capt. Jeff Davis, a Pentagon spokesman, said the leaflets told the pro-government forces to leave the established protected zone, which is about 55 kilometers around an area where U.S. and coalition forces have been operating.

U.S. officials have said the weapons deliveries will include heavy machine guns, ammunition, 120mm mortars, armored vehicles and possibly TOW anti-tank missiles. They said the U.S. would not provide artillery or surface-to-air missiles.

Less than two weeks ago, the U.S. bombed Iranian-backed troops who were in that same area of Syria and didn’t heed similar warnings to leave.

According to Syrian and U.S. officials, the bombing killed several soldiers and destroyed vehicles and other weapons and equipment.

Davis said the U.S. has seen the militias operating in the desert around Tanf. The area has been considered a “deconflicted” zone under a U.S.-Russian understanding.

“Hundreds” of pro-government forces are in the region, Davis said, but he was unsure how many are actually inside the zone.

Pentagon officials said they were not certain if those troops are Syrian, Iranian, Hezbollah or from other militias fighting on Assad’s behalf. At the Tanf military camp near the Jordanian border, U.S. special operations forces have been working with a Syrian opposition group in operations against IS.