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Russian participation in the Syrian civil war appears to be on the rise amid reports of more ground troops entering Syria and new Turkish claims of Russian harassment.

UN Secretary General Jens Stoltenberg said during an appearance on CNN that the number of Russian ground troops in Syria is building while also confirming that a second Russian airplane entered Turkish airspace over the weekend.

“So we have seen a substantial military buildup by Russia in Syria, both in the air with the combat planes and air defense systems, but also an increasing number of ground troops,” he told the station. “In addition to that, they have deployed naval assets, a large number of naval assets close to the Syrian shores. And they continue to do so.”

WATCH: Russian air power giving the Syrian army an advantage

The Russians have said they will be using “volunteer” ground forces in the Syrian conflict, according to the New York Times.

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The European power already has more than 600 troops in Syria, the Times reports.

Over the weekend, Russian planes invaded Turkish airspace in a pair of separate incidents on Saturday and Sunday.

The NATO alliance on Monday denounced Russia for “irresponsible behaviour” for allowing its warplanes to twice cross into Turkish airspace, once on Saturday and once on Sunday. Turkey warned that any future aerial intruder would be treated like an enemy. The Russians acknowledged having intruded once on Saturday, calling it accidental, but denied doing it again Sunday.

READ MORE: NATO denounces Russia for violation of Turkish airspace

“It’s unacceptable to violate the airspace of another country,” UN Secretary General Jens Stoltenberg told reporters on Tuesday. He said NATO is expressly worried that such acts by the Russians could have unforeseen consequences.

The Russian-Turkish issue also seems to be escalating as the Turkish army also claims a squadron of their jets were placed on radar lock-on for several minutes by an unidentified plane near the Syrian border Monday.

“Eight Turkish F-16 jets carried out reconnaissance flights over the Turkish-Syrian border (on Monday) and during this mission our jets were held on radar lock by an unidentified MIG-29 plane for a total of four minutes and 30 seconds,” the Turkish army said in a statement as reported by French news service Agence French Press.

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READ MORE: Turkey scrambles jets after Russian warplane violates airspace

Radar lock-on is designed to allow missile systems to automatically keep track of a moving target.

In a separate incident, the Turks said the F-16s were “harassed by missile systems deployed in Syria for a total of four minutes and 15 seconds.”

Russian officials say they have more than 50 warplanes and helicopters taking part in the open-ended air operations.

With Files from the Associated Press