ISTANBUL - Turkish President Recep Tayyip Erdogan on Friday expressed alarm over reports of a build-up of Russian troops in northern Syria near the Turkish border, saying such movements will not be tolerated.

Britain-based monitor the Syrian Observatory for Human Rights had said that Russia had sent a number of engineers to the Syrian border town of Qamishli to strengthen the runway and increase the capacity of an airport there.

Russia’s reported move into Qamishli comes as Ankara and Moscow are experiencing their biggest crisis in years over the shooting down of a Russian war plane by Turkey on November 24.

“We have said this from the beginning: we won’t tolerate such formations (in northern Syria) along the area stretching from the Iraqi border up to the Mediterranean,” Erdogan told reporters after Friday prayers in Istanbul.

“We maintain our sensitivities on this issue,” said Erdogan, citing reports that Russia had deployed some 200 soldiers and adding he would raise the issue Saturday in talks with visiting US Vice President Joe Biden.

Observers have said that Russia, which has for years been at loggerheads with Turkey over the Syrian conflict, may want to refit the airport as a Russian base, as happened in Hmeimim in Latakia province. Qamishli lies just south of the Turkish border town of Nusaybin.

“I can say that Turkey is closely watching every military movement on its borders and especially the border with Syria,” the government source told AFP, asking not to be named. The Turkish army has already reinforced security by digging trenches in the border zone, the Hurriyet daily said. Top Russian military officials, including figures from the GRU military intelligence service, had already visited Qamishli, it added.

The Kremlin and Iran are the chief remaining allies of President Bashar al-Assad who Turkey wants to see ousted as the key to ending Syria’s almost five year civil war.

Turkey has repeatedly expressed alarm about Russia’s deployment of troops to Syria which Moscow says is aimed at fighting militants but is widely seen as buttressing the Assad regime.

Education Minister Nabi Avci, speaking in parliament, claimed Russia’s forces in Qamishli were working with both the regime as well as Kurdish militia of the Democratic Union Party (PYD).

The People’s Protection Units (YPG) have established control over much over the northern Syrian border region in the past months after pushing out Islamic State (IS) militants.

But Ankara accuses the PYD and YPG of being the Syrian branch of the outlawed Kurdistan Workers Party (PKK) and has been alarmed by an apparent tightening of ties between Moscow and the Kurds after the plane downing.

“For us, there is no difference between PYD, YPG, PKK or Daesh (IS),” said Erdogan.

“We will discuss this with Biden tomorrow... I hope that this joint stance will be aimed at preventing this wrong Russian formation in northern Syria,” he added.

Meanwhile, NATO member states are discussing a request to use the alliance’s AWACS air surveillance aircraft in the battle against the militant Islamic State group, an official said Friday.

Any AWACS commitment would mark a departure for the alliance which up to now has had no direct role in combatting IS but is increasingly concerned by the threat it poses on its southern flank.

“We can confirm there has been a request for concrete support from NATO to the anti-ISIL effort in the form of NATO AWACS surveillance planes,” a NATO official said.

“This request is under discussion by allies,” the official said.

“Any decision would be in line with the Wales summit declaration which indicates NATO’s readiness to support the bilateral efforts of allies,” the official said, referring to a 2014 meeting of alliance leaders.

Diplomatic sources said the request came from the United States which leads an anti-IS coalition of more than 60 countries, including most of NATO’s 28 member states.

NATO declined to comment on the source of the request.

US officials at NATO were not immediately available to comment.

NATO has few military assets of its own, providing the umbrella command structure for the 28 allies, but in the 1980s they agreed to establish an AWACS (Airborne Warning and Control System) unit to counter the then Soviet threat.

The AWACS unit comprises more than 20 planes capable of monitoring activity, principally in the air but also on the ground, within a radius of some 400 kilometres (260 miles) to warn of threats and coordinate a response.