ANKARA, Turkey (AP) — Turkey’s president urged Russian President Vladimir Putin in a telephone call Friday to “restrain” the Syrian government and halt the humanitarian crisis unfolding in northwestern Syria as Damascus wages a military offensive against the last rebel stronghold in the country.

Recep Tayyip Erdogan also called for the full implementation of a 2018 Turkish-Russian cease-fire agreement for Idlib province, which collapsed following the Russia-backed Syrian government advance, a statement from Erdogan’s office said.

The two leaders expressed their commitment to “all agreements,” Erdogan’s office said, but did not elaborate.

Putin expressed “serious concern with continuing aggressive actions by extremist groups (in Idlib),” the Kremlin said in a statement. “The necessity of unconditional respect for the sovereignty and territorial integrity of Syria was underlined.”

The discussion between the two leaders came amid mounting tensions as the Syrian government pushes ahead with an offensive that has displaced hundreds of thousands of people. The offensive has tested ties between Turkey and Russia, who had been working together closely in Idlib, despite backing opposing sides in Syria’s nine-year civil war.

Erdogan this week warned of an “imminent” operation against Syria to compel it to pull its forces back behind Turkish positions by the end of February. He said earlier Friday that the phone call with Putin would determine Turkey’s position on Idlib.

Two Turkish soldiers were killed in an airstrike in Idlib on Thursday after an attack by Ankara-backed opposition forces that targeted Syrian government troops. The deaths came after Erdogan threatened to expand his nation’s involvement in Syria if another one of his troops were hurt.

Earlier, Erdogan also held a joint call with French President Emmanuel Macron and German Chancellor Angela Merkel in which he said that “the Syrian regime and its backers’ aggression in Idlib must be stopped.” He also emphasized the need for “strong support and concrete action” to halt the unfolding humanitarian crisis, his office said.

It followed a similar call by Merkel and Macron to Putin on Thursday, during which they proposed urgent talks with Erdogan to ease tensions. Erdogan told reporters the European leaders had proposed a four-way meeting in Istanbul on March 5, but that Putin had not responded to the offer.

As Erdogan pressed ahead with diplomatic efforts on Idlib, Turkey’s Defense Minister Hulusi Akar, accompanied by military commanders, traveled to the Turkish-Syrian border to inspect Turkish troops. He spoke with troops deployed in Idlib telling them he believed they would “succeed” in their duties in Syria.

Also on Friday, Russian warplanes struck rebel-held areas in the region as insurgents and government forces exchanged artillery and mortar fire in other parts of Idlib province, the last rebel stronghold in the country, opposition activists said.