Turkish President Recep Tayyip Erdogan and his Russian counterpart, Vladimir Putin, have underlined the importance of Syria’s sovereignty and achievements gained in the Astana peace negotiations for a political solution in war-torn Syria.

The two leaders discussed developments in Syria in a phone conversation on Tuesday, as nine rounds of crisis resolution talks between the Syrian government and the foreign-backed opposition, mediated by Iran, Russia and Turkey, have so far been held in the Kazakh capital over the past months.

The trio has been mediating the negotiations in Astana since January 2017, a month after they joined efforts and brought about an all-Syria ceasefire. The three states act as the guarantors of the truce.

According to Turkey's official Anadolu news agency, Erdogan and Putin emphasized the sovereignty and territorial integrity of Syria, stressing that both Damascus and the so-called opposition groups should act constructively for the progress of peace process within the UN framework.

The Astana peace process has significantly helped reduce the violence in Syria through the formation of four de-escalation zones in the country, setting the stage for Syria’s warring sides to focus on negotiations on the political future of their homeland.

The negotiations in Astana have been going on along with another series of talks held in Geneva, Switzerland, and brokered by the UN. Previous rounds of negotiations under the auspices of the world body over the past five years have failed to achieve tangible results.

Erdogan also reiterated Ankara’s strong opposition to the inclusion of groups purportedly linked to the “terrorist” Kurdistan Workers' Party (PKK)’s Syrian offshoot People's Protection Units (YPG) in the political process.

Ankara views the YPG as the Syrian branch of the PKK that has been fighting for an autonomous region inside Turkey since 1984.

Back in January, Ankara launched the ongoing Olive Branch offensive against the purported positions of the YPG in Syria’s northwestern enclave of Afrin without granting any permission from the Syrian government, drawing harsh condemnation from Damascus.

Syria has been gripped by foreign-backed militancy since March 2011.