Russian President Vladimir Putin (R) and his Turkish counterpart Recep Tayyip Erdogan shake hands during a news conference following their talks in Sochi, Russia, Sept. 17, 2018. Photo: Alexander Zemlianichenko / Pool via Reuters.

President Recep Tayyip Erdogan will seek steps from Russia‘s Vladimir Putin to safeguard Turkish troops in the face of an offensive by the Syrian army in the country’s northwest when the two leaders meet on Tuesday, a senior Turkish official said.

Erdogan, who is making a one-day visit to Russia, told Putin last week that attacks by Syrian President Bashar al-Assad’s forces were causing a humanitarian crisis and threatened Turkey‘s national security.

The official told Reuters that the security of Turkish soldiers in Syria would be one of the key topics at the meeting. The meeting was due to start at 1:30 p.m. (1030 GMT), with a joint statement to be issued at 4 p.m. (1300 GMT).

“We expect Russia to use its influence over the regime on this matter. If there is even the smallest attack on Turkish soldiers, we will retaliate against this,” the official said.

Syrian troops have encircled rebels and a Turkish military post in northwest Syria in an offensive to reclaim territory and towns the government lost early in the war. Turkey has supported some rebel factions in the northwestern Idlib region, while Russia and Iran back Assad.

The military observation post near the town of Morek is one of 12 that Ankara established in northwest Syria under a deal with Moscow and Tehran two years ago to reduce fighting between Assad’s forces and rebels.

“Any step or attack that would violate the agreement should be avoided, but unfortunately we see examples of these in recent times,” the official said. “We expect Putin to take steps that will alleviate the problem there.”

Erdogan and Putin hold frequent talks and have forged close ties focused on energy and defense cooperation. In July, Turkey began taking delivery of Russian S-400 missile defense systems – a move that strained ties with Ankara’s NATO ally the United States.

Turkish Defence Minister Hulusi Akar said on Monday that delivery of the second battery of the S-400 system would begin on Tuesday.

As well as putting Turkish troops in the region in the firing line, the advances of Assad’s forces have threatened Ankara’s hopes of preventing a fresh wave of refugees – including fighters – on its southern border.

The United Nations says more than 500,000 people have been uprooted since the Syrian army began its offensive in late April, most of them escaping deeper into the rebel bastion and toward the border. Turkey opened its border at the start of the Syrian conflict in 2011 and now hosts 3.6 million Syrian refugees.

“The necessary measures need to be taken to prevent a migrant wave from there to Turkey. Measures should be taken against any problems that may arise on this issue,” the official also said.