(Bloomberg) -- Turkish President Recep Tayyip Erdogan is expected to urge the presidents of Russia and Iran to restrain Syria’s planned offensive on the last major rebel holdout, an assault that threatens to touch off the biggest migrant crisis since Syrian refugees flooded Europe in 2015.

Erdogan’s determination to block entry to Syrians fleeing an expected Russian-backed offensive on northwestern Idlib province will figure centrally in his meeting with Russian President Vladimir Putin and Iran’s President Hassan Rouhani in Ankara on Monday. The Turkish leader has demanded the European Union do more to ease his country’s financial burden.

The meeting will also give Putin and Rouhani an opportunity to discuss the drone attacks on Saudi oil facilities that knocked out about 5% of global supplies and escalated already soaring tensions in the region. Iran-backed Yemen rebels claimed responsibility, but the U.S. has blamed Iran, which has denied the charge.

Pacifying Idlib would allow President Bashar al-Assad to consolidate control over all but the oil-rich northeastern part of Syria, setting the stage for reconciliation and reconstruction by ending the worst of the fighting. However, Erdogan is worried an Idlib offensive will set off a new wave of migrants into his country, which is already hosting more than 3.6 million Syrians.

Turkey says 500,000 Syrians have already hunkered down just across the border, and as fighting in Idlib has intensified in recent weeks, the number of migrants entering Europe from Turkey has grown. If the fighting give way to an all-out offensive, Turkey has said it will block fleeing Syrians from entering.

Still smarting after the ruling party’s loss of Turkey’s two biggest cities in local elections in June, and facing mounting criticism over the cost of hosting Syrians, Erdogan wants to find a way to send at least a million refugees back.

He also has other considerations in mind: He’s trying to leverage the looming migrant crisis to persuade the U.S. to push American-backed Syrian Kurdish fighters who control northeastern Syria away from Turkey’s southern border.

Erdogan accuses the separatist Kurdish YPG militia of having designs on Kurdish enclaves in Turkey, too, and wants to weaken their hopes for an autonomous enclave in northern Syria. He’s portraying their presence there as the biggest obstacle to repatriating Syrian refugees in a planned buffer zone along the shared frontier.

“Our expectation from the U.S. in our struggle against terrorism is to support us in creating a safe zone where refugees can return,” Erdogan told an audience that included U.S. Commerce Secretary Wilbur Ross on Sept. 10.

Erdogan refers to the Kurdish YPG militia, which played an instrumental role helping the U.S. defeat Islamic State, as “terrorists” because of their ties to the separatist Kurdistan Workers’ Party, or PKK, which has battled Turkish security forces for decades. The PKK is also labeled a terrorist organization by the U.S. and European Union.

“Erdogan’s plan to relocate Sunni refugees along the border area is part of his push to confront Kurdish influence on Turkey’s doorstep,” said Nihat Ali Ozcan, a strategist at the Economic Policy Research Foundation in Ankara.

Erdogan wants the U.S. to lengthen and deepen a planned buffer zone in northeastern Syria that would be off-limits to the YPG, and wants to discuss the issue with Trump at the United Nations General Assembly in New York this month. He’s already threatened an offensive against the Kurdish militia if the U.S. doesn’t relent.

Amid a near-constant battle of wills with the U.S., Turkey’s partner in NATO, Erdogan has drawn closer to Russia.

Ankara recently bought a Russian missile-defense system, and has mooted the idea of ordering advanced fighter jets from Moscow, despite the threat of U.S. sanctions that could send Turkey’s fragile currency into another dive. A Russian Su-35 warplane flew over Istanbul on Sunday before its display at a defense fair Tuesday, showing Russia’s interest in selling the jet to Turkey.

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Erdogan’s administration suspects Washington ultimately supports the YPG’s aspiration for some form of Kurdish self-rule, and is using the group to defend American interests as Syria’s eight-year civil war nears an end.

“The distrust and skepticism run very high” but both Ankara and Washington know they must salvage their ties, said Kilic Bugra Kanat, a Washington-based research director at Turkish think-tank SETA.

To contact the reporters on this story: Selcan Hacaoglu in Ankara at shacaoglu@bloomberg.net;Firat Kozok in Ankara at fkozok@bloomberg.net

To contact the editors responsible for this story: Onur Ant at oant@bloomberg.net, ;Lin Noueihed at lnoueihed@bloomberg.net, Amy Teibel, Paul Abelsky

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