Turkey and its rebel proxies have stepped up attacks against the Syrian Kurdish People’s Protection Units (YPG) in and around Afrin in recent months, both from within Syria but also through sustained shelling from the Turkish side of the border.

The moves were followed by the announcement by Turkish presidential spokesman Ibrahim Kalin that Turkey, Iran and Russia are working on de-escalation zones in Syria that will also involve the United States. Kalin said that Turkey and Russia could jointly deploy forces to Idlib province, which is dominated by the al-Qaeda-linked Hayat Tahrir al-Sham. The presence of Turkish troops in Idlib would add to pressure on Afrin, the mainly Kurdish enclave that has long been a target.

Reuters quoted a rebel from a Turkish-backed group as confirming the deployment. “Turkish forces are now inside Syria. … The forces are huge reinforcements that have been entering since last night,” said Mustafa Sejari of Liwa al-Mutasim.

The Syrian Observatory for Human Rights, a London-based monitoring group, reported on Wednesday that Turkish forces had crossed the Syrian border and were headed toward the rebel-held border town of Azaz, one of the main logistical hubs for Turkish operations inside northern Syria.

Fears of a Turkish-backed Syrian rebel attack against Syrian Kurdish forces that could undermine the US-led campaign against the Islamic State are growing amid reports that Turkey sent new military reinforcements into northern Syria, including troops and supplies.

Commanders from the Syrian Democratic Forces (SDF), the YPG-led umbrella group that is the United States’ top ally in the fight against IS, told Al-Monitor that the Turkish-backed moves were focused on the town of Tell Rifaat, which lies 40 kilometers (25 miles) north of Aleppo and was seized from Turkish-backed rebels in February 2016. “They want to use Azaz as a springboard,” an SDF official told Al-Monitor via WhatsApp. The official said that a Turkish-inspired attack on Afrin would disrupt the ongoing campaign to liberate Raqqa but declined to elaborate. The town is critical to the YPG’s long-running efforts to connect the territories the group controls to the east of the Euphrates River with Afrin.

Turkey sent hundreds of troops into the northern Syrian town of Jarablus last August, mainly to disrupt these plans. The YPG has repelled several attempts by the Turkish-backed groups to reclaim Tell Rifaat but the shifting dynamics of the Syrian war may now be working against the YPG — at least in Afrin, where it has long relied on Russia for protection. The United States has made it clear that it will not intervene in the area, which lies outside its zone of influence, and has refused to help the YPG join its cantons, mainly out of deference to Turkey.

The YPG enjoys US protection to the east of the Euphrates, where the US-led coalition has been steadily rolling back the jihadis and have now embarked on a campaign to free Raqqa. The Russians have been operating chiefly to the west of the river to help the regime fight its rebel opponents.

A coalition spokesman responding via email to Al-Monitor's queries said, "The coalition is aware of skirmishes between Turkish-supported forces and the SDF in northern Syria. The coalition does not speak on behalf of our Turkish or SDF partners, though. The coalition's mission is to defeat [IS] in Iraq and Syria. We call on all parties in the region to remain focused on the fight to defeat [IS], which is the greatest threat to regional and worldwide peace and security."

However, the de facto partitioning of US and Russian zones of influence are coming unstuck as IS gets rolled back and both sides jockey for control over the strategic southeastern desert region bordering Iraq. The Syrian Kurds, who have cannily balanced relations with both, are stuck in the middle. Turkey and Iran, which are on opposing sides of the Syrian conflict but share fears about Kurdish gains, are seeking to exploit these divisions.

Syrian Kurdish officials have accused Russia of wielding the threat of Turkish aggression against Afrin to not only pressure them into letting the regime move back into the area but also to help advance the regime’s interests in and around Deir ez-Zor.

Arzu Yilmaz, a researcher at the American University of Kurdistan in Dahuk, told Al-Monitor, “Turkey cannot move against the YPG in Afrin without Russia’s consent, this is crystal clear.” She added, “The clash of interests between Russia and the United States is obviously squeezing the YPG."

Tensions boiled over on Sunday, when the United States shot down a regime jet after it allegedly dropped bombs near SDF positions in the western Raqqa countryside. The action marked the first time the United States downed a Syrian aircraft since the start of the Syrian war.

The move provoked angry reactions from the regime and its top ally Russia, both of whom want to prevent the SDF from carrying their campaign to wrest Raqqa on to the regime-held city of Deir ez-Zor. The city, the administrative capital of Syria’s oil-rich eastern desert, is under siege by IS. It is also of strategic value to Iran, which is seeking a permanent foothold in the region.

A Syrian Kurdish source with close links to the local officials who spoke to Al-Monitor on strict condition of anonymity insisted that the Syrian Kurds could reverse the situation in Afrin. The pro-YPG administration controls much of Qamishli, where the Syrian regime maintains a small presence. More critically, regime forces remain in charge of Qamishli's airport, a vital hub for its Iranian allies. The Iranians use the airport to ferry their troops and weapons to Deir ez-Zor.

The source said that the YPG could cut a deal with the Russians to take over the base, presumably in exchange for relieving Turkish pressure on Afrin. “The Russians are not so happy with Iran, either, because of its support for the regime,” the source said. “Iranian support makes the regime even more uncompromising. It is provoking tensions with America and making deals with Turkey behind Russia’s back, that is why.”