Turkey has been the biggest military and political ally of the Syrian opposition in the war-torn country’s northwest.

Speaking to reporters in Istanbul on Monday ahead of his departure for a three-day visit to Saudi Arabia, Qatar and the United Arab Emirates, Erdogan said Turkey has never “shut the door” to discussions with the Syrian government.

Turkey has been the biggest military and political ally of the Syrian opposition, which controls the last rebel-held bastion in the country, which is in northwest Syria on the Turkish border.

Ankara has set up dozens of bases and deployed thousands of troops in northern Syria, preventing the Russian-backed Syrian army from re-taking the region. Turkey has also been a base for Syrian opposition groups since 2011.

“We can hold a four-party summit [with Syria, Russia and Iran], and I am also open to a meeting with Assad. What matters here is their approach towards us,” Erdogan told journalists.

However, Damascus’s condition of a complete withdrawal of Turkish forces for such a meeting is “unacceptable”, he said.

Erdogan first said this year that he might meet al-Assad as part of a new peace process, but al-Assad said in March that there was no point in a meeting with Erdogan until Turkey’s “illegal occupation” ended.

Turkey has said its military operations in Syria have been necessary to secure its southern border. It is trying to remove fighters with the YPG, the Kurdish People’s Defence Units, which Ankara has said is the Syrian branch of the PKK, the Kurdistan Workers’ Party. The PKK has fought a war against the Turkish state since 1984 – a conflict that has led to tens of thousands of deaths.

“We are fighting against terrorism there. How can we withdraw when our country is under continuous threat from terrorists along our border? … We expect a fair approach,” Erdogan said.

The defence ministers of Turkey and Syria met late last year for the highest level talks between the two neighbours since the 2011 Arab Spring uprisings plunged Syria into war and put it and Turkey at odds.

The foreign ministers of the two countries also met in Moscow in May ahead of Turkish elections as part of talks overseen by Russia.

While tensions between the two countries remain, Arab states have been looking to normalise ties with al-Assad.

The restoration of ties with Damascus quickened its pace after the deadly February 6 earthquakes in southern Turkey and northwestern Syria and the Chinese-brokered re-establishment of ties between Saudi Arabia and Iran, which had backed opposing sides in the Syrian conflict.

On Sunday, Iraq’s prime minister held talks with al-Assad in Damascus during the first trip of its kind to the war-torn country since the Arab Spring began.

The decision to allow Syria back into the Arab League in May sparked anger among many residents of opposition-held areas in Syria and members of the country’s political opposition, who see it as a vindication of the government’s attacks against them during a 12-year war.