Turkish President Recep Tayyip Erdogan on Saturday discussed with US counterpart Donald Trump ways to end the crisis in Idlib, Syria's last major rebel bastion, and condemned Damascus' attacks there, the Turkish presidency said.

"Stressing that the regime's most recent attacks are unacceptable, the president and Trump exchanged views on ways to end the crisis in Idlib without further delay," the presidency said in a statement after the two leaders spoke on the phone.

The situation in Syria has become more tense as Syrian President Bashar al-Assad intensifies his assault on Idlib backed by Moscow air power. Hundreds of civilians have been killed while 800,000 Syrians have been forced to leave their homes to flee the offensive since December, according to the United Nations.

Turkey has 12 observation posts in Idlib as part of the 2018 Ankara-Moscow agreement made in southern city of Sochi. Erdogan threatened to attack Damascus if regime forces did not go back behind the borders of the Sochi deal by the end of February after 14 Turks were killed by Syrian regime shelling in Idlib this month.

Relations between Turkey and the United States have been strained over multiple issues including Syria but it appears the Americans are trying to capitalise on the tensions over Idlib between Ankara and Moscow.

US special envoy for Syria James Jeffrey came to Ankara earlier this week, and voiced Washington's support for Ankara's "legitimate" interests in Syria and in Idlib. But Turkish Foreign Minister Mevlut Cavusoglu earlier on Saturday insisted that Turkey-Russia relations "shouldn't" be affected by differences of opinion over Syria.

"The situation in Idlib will not affect the S-400 agreement," Cavusoglu said, referring to Ankara's purchase of the Russian air defence system criticised by Washington. The US has threatened to hit Turkey with sanctions over the S-400 deal.

The Turkish presidency also said Erdogan and Trump discussed the situation in war-torn Libya and "underlined the importance of restoring peace and stability."