Russia is setting up a military base in northwestern Syria in agreement with the Syrian Kurdish YPG militia that controls the area and will be training YPG fighters as part of the fight against terrorism, the militia's spokesman said on Monday.

YPG spokesman Redur Xelil told Reuters the agreement with Russia was concluded on Sunday, and that Russian troops had already arrived at the position in the northwestern region of Afrin with troop carriers and armoured vehicles.

The Russian presence... comes in the framework of cooperation against terrorism and to help train our forces on modern warfare - Redur Xelil, YPG

Separately, Russia's defence ministry denied the reports that the YPG militia had reached a deal with Moscow for a new military base in northwestern Syria.

"There are no plans to deploy new Russian military bases on the territory of Syria," a defence ministry statement on Monday read.

It added that a section of its "reconciliation centre", which Russia says helps negotiate local truces between the warring sides in Syria, was located in Aleppo province near Afrin for the prevention of ceasefire violations.

If it is confimred, the move will likely anger neighbouring Turkey. Ankara views the YPG as an extension of the Kurdistan Workers' Party (PKK), which is waging an insurgency inside Turkey.

"The Russian presence... comes in agreement between (the YPG) and the Russian forces operating in Syria in the framework of cooperation against terrorism and to help train our forces on modern warfare and to build a direct point of contact with Russian forces," Xelil said in a written statement.

"It is the first of its kind," he added.

Turkey has launched a cross-border offensive along a section of the Turkish-Syrian frontier to prevent further gains by the YPG, which controls swathes of northeastern Syria and the Afrin pocket of northwestern Syria.

The YPG is also allied to the United States in the fight against Islamic State in Syria, and is playing a major part in the US-backed offensive against Islamic State's urban stronghold of Raqqa, further east.

"The agreement came into force today," Xelil said, declining to say how many Russian troops had arrived in Jandaris, the place where the base is being established.

Jandaris has previously been shelled by Turkish forces from across the nearby frontier, Xelil added.

The YPG makes up the bulk of the Syrian Democratic Forces, a US-backed alliance of Kurdish and Arab fighters that has seized swathes of territory from IS in Syria.

The SDF receives equipment, weapons and air support from the US-led coalition, and is backed by several hundred Western special operations forces in an advisory role.

Russia, meanwhile, is a long-term backer of Syrian President Bashar al-Assad, but had recently worked closely with rebel supporter Turkey to try to end the six-year war in Syria.

Aaron Stein, a senior fellow at the US-based Atlantic Council, said Russia was now partnering with the Kurds as they had become an important player in Syria.

"The Kurds are now the most consequential non-state actor in Syria, alongside al-Qaeda... They will have a huge say over the future of Syria," he said.