Istanbul (AFP) - 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 would not be tolerated.

"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.

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 maintain our sensitivities on this issue," added Erdogan, citing reports that Russia had deployed some 200 soldiers and stating that 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.

- 'Prevent this formation' -

The Turkish army has already reinforced security by digging trenches in the border zone, the Hurriyet daily said.

It quoted Turkish security sources as saying that a Russian military delegation, headed by a general and including members of the Russian military intelligence service GRU, had flown into Qamishli on January 16 to inspect the airport.

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.

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Turkey has repeatedly expressed alarm about Russia's deployment of troops to Syria which Moscow says is aimed at fighting jihadists 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) jihadists.

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.

Hurriyet also claimed a delegation from Russia's Federal Security Service (FSB), the PYD and the Assad regime was now in Qamishli.