In a major boost for forces fighting Islamic State extremists in Syria but likely to stir controversy with NATO ally Turkey, the United States on Monday began airdropping pallets of weapons and ammunition to a Syrian Kurdish militia and allied Arab forces in northern Syria.

“They started dropping the arms in Rojava early this morning,” said Polat Can, the spokesman for the Democratic Union Party (PYD), the Kurdish political party whose armed wing, with the help of U.S. bombing, has pushed the Islamic State from as much as 6,800 square miles of northern Syria. Rojava, or “west Kurdistan,” is the name the PYD uses to refer to northern Syria’s Kurdish areas.

Meanwhile, Russian aircraft continued to bomb targets in Syria’s west, far removed from the main operating areas of the Islamic State, while the United Nations’ special envoy for Syria, Staffan de Mistura, stepped up his efforts to find a diplomatic solution to the conflict, setting visits to Moscow and Washington. He said his first priority was to make certain that Russian military intervention doesn’t result in a spreading conflict.

The Pentagon confirmed that C-17 transport aircraft had dropped 45 tons of arms in 100 pallets to groups inside northern Syria. But it said the initial drop, which it said occurred late Sunday night, was to benefit “Arab groups,” a nod to Turkish concerns about U.S. support for the PYD’s Popular Protection Units, or YPG, militia.

Can said the airdrops are expected to continue for days.

The drop came just two days after the Pentagon announced that it had ended its ill-starred $500 million program to train and equip vetted Syrians to fight the Islamic State and said that the money that remained would be used to supply weapons to armed groups already in Syria that had had success combating the Islamic State.

Can said the weapons dropped in “Rojava” included assault rifles, mortars and ammunition – but no TOW anti-tank missiles nor anti-aircraft weapons. He said the Kurdish forces would distribute weapons to Arab units affiliated with the YPG.

“Everyone will take arms. We believe in sharing,” he said in an interview with McClatchy, adding that the YPG’s ability to provide arms “is why some Arab tribes are joining us.”

It’s just short of a year since the United States last dropped weapons to YPG, which at the time was battling to beat off an Islamic State offensive against the town of Kobani. The YPG prevailed, with the help of hundreds of American airstrikes. Turkey, a U.S. NATO ally and a major player in the region and which views the YPG as a terrorist organization, objected bitterly.

The YPG is the Syrian affiliate of the Kurdistan Workers Party or PKK, with which Turkey is currently fighting in southern Turkey and in Iraq.

The very fact that the U.S. could not deliver the arms overland from the territory of two allies, Turkey or Iraq’s Kurdistan Regional Government, illustrates the controversy surrounding the U.S. decision to supply the YPG.

Turkey views the YPG’s stated ambition of creating a contiguous Kurdish-run entity in northern Syria as a threat to its own security. Meanwhile, the largely autonomous Kurdistan government in Iraq has strained relations with the YPG militia and often holds up its supplies over the land border to Syria.

The quantity of arms may add to the strains. The Obama administration has said much of what remains of the money appropriated for the train-and-equip program will go to groups in northern Syria, a huge amount for a force that numbers an estimated 20,000.

Under the plan, a YPG officer will be in overall command of the Kurdish-Arab fighting force, which is calling itself the Syrian Democratic Forces. Can announced the creation of the alliance Sunday, just as the airdrops were starting.

U.S. officials hope the YPG will now turn its attention to Raqqa, the Syrian city that is the defacto capital of the Islamic State, which lies just 60 miles south of Tal Abyad, a border town the YPG seized from the Islamic State in June, with U.S. help.

But PYD spokesman Can said the Kurdish group’s first priority is to link the Kurdish enclave of Afrin, northwest of the Syrian city of Aleppo, with Kobani, the Kurdish enclave northeast of Aleppo. That would mean clearing the Islamic State from villages along 60 miles of the Turkey-Syria border, in particular the border town of Jarablus.

“Our prime and most important goal is to liberate Jarablus and to connect Kobani with Afrin,” Can told McClatchy. Capturing Raqqa, a mostly Arab city, is “not really” a PYD objective, he said. “Not for now,” he said.

But the capture of Jarablus and the linking of Afrin with Kobani is likely to be seen as a threat to Turkey, whose objection to Kurds control that last stretch of border was one reason a Kurdish push against the Islamic State has stalled in recent weeks.

“The Turks are very unhappy,” Can said, though he added that at the end of the day, the outcome in northern Syria is “a Syrian issue, not a Turkish issue.”

Turkish officials have said Turkey will not permit the YPG to establish a contiguous link across northern Syria, but how Turkey will prevent that is not clear. Turkey is preoccupied by the final six weeks of a controversial parliamentary election campaign and reeling from a suicide bombing Saturday that killed nearly 100.

There was no immediate Turkish reaction to the airdrops. A Turkish government official interviewed in Ankara last week told McClatchy that Turkey cannot and will not let a linkage of the Kurdish enclaves happen.

In noting that the first airdrop was to “Arab groups,” a Pentagon spokeswoman said “we share the concern of our Turkish partners over the sensitivity of expanding Kurdish control into traditionally non-Kurdish areas in Syria.”

Meanwhile, Russia continued its bombing campaign in western Syria, announcing that its aircraft had carried out 55 combat sorties “engaging 53 ISIS objects.” But the locations it stated, Homs, Hama, Latakia and Idlib provinces, are locations held by rebel forces and not by the Islamic State.

De Mistura said he would meet with Russian Foreign Minister Sergey Lavrov on Tuesday in Moscow, then travel to Washington.

James Rosen in Washington and special correspondents John Zarocostas in Geneva and Zakaria Zakaria in Sulaymaniyah contributed.