Turkey's Erdoğan to pay an official visit to Russia on August 27

Turkish President Recep Tayyip Erdoğan will make a one-day official visit to Russia on August 27, the Turkish presidency said on Friday, hours after he held a phone call with Russia's Vladimir Putin to discuss developments in northwestern Syria.

In the phone call, Erdoğan told Putin that an offensive by forces of Bashar al-Assad, backed by Moscow, was causing a humanitarian crisis and posed a threat to Turkey's national security.

The surprise visit comes as the Assad regime forces made advances in the northwest of Syria and upped the stakes with Turkey in its months-long offensive backed by Russia.

Turkey is a vocal opponent of Bashar al-Assad in Damascus and instead backs rebels fighting for his ouster.

But Ankara last year struck a deal with Moscow to protect the Idlib province from a massive government offensive.

The Syrian regime and its allies, however, have consistently broken the terms of the cease-fire, launching frequent attacks inside the zone.

The de-escalation zone is currently home to some 4 million civilians, including hundreds of thousands displaced in recent years by regime forces from throughout the war-weary country.

Turkish officials on Monday "strongly" condemned an air strike on its military convoy heading through Idlib province, saying it was a violation of agreements with Syria.

Ankara blamed "regime forces" for the attack which it said killed three civilians, and the Britain-based Syrian Observatory for Human Rights said the attack was carried out by Syrian and Russian air forces, and was aimed at hindering the convoy's advance through Idlib province.

Putin and Erdoğan on Friday agreed to "activate mutual efforts" to ease the situation in Idlib during a phone call, according to the Kremlin.

"They agreed to activate mutual efforts with the goal of liquidating the terrorist threat coming from this region," during a phone call initiated by Erdogan, it said.

The Turkish presidency said "ceasefire violations in Idlib by the regime, and its attacks have led to a grave humanitarian crisis."

"These attacks damage the efforts to regulate the Syrian conflict," it said.

Since late April, Syria and Russia have upped their bombardment of the Idlib region of some three million people, killing more than 860 civilians.

Syria has been locked in a vicious civil war since early 2011, when the Bashar al-Assad regime cracked down on pro-democracy protests with unexpected ferocity.

Since then, hundreds of thousands of people have been killed and more than 10 million others displaced, according to UN officials.