We don't believe Russia on Turkey incursions - Nato
Nato's secretary-general has rejected Moscow's claim that its military incursion into alliance airspace over Turkey was not intentional or important.
Turkey's military meanwhile said several of its jets patrolling the border with Syria were again targeted by Russian planes and surface-to-air missile systems.
Russian warplanes continued pounding targets in Syria, where the Kremlin has come to the aid of beleaguered ally President Bashar Al-Assad.
Nato chief Jens Stoltenberg said breaches of Turkish airspace by Russian planes were "very serious" - even dangerous.
"It doesn't look like an accident, and we've seen two of them over the weekend," he added.
The latest Russian air strikes in Syria, in co-operation with Syrian jets, struck targets in rural areas of the northern Aleppo province, targeting the towns of al-Bab and Deir Hafer, Syrian state TV reported.
Both towns are controlled by Isil.
The official also said Isil bases were targeted in Palmyra and surrounding areas in the central Homs province, destroying 20 vehicles, three arms depots and three rocket launchers.
A spokeswoman said Nato generals would be contacting their Russian counterparts about the violation of Turkish airspace.
"It's unacceptable to violate the airspace of another country," Mr Stoltenberg said.
Turkey's military said yesterday that eight Turkish F-16 jets patrolling the Turkish-Syrian border were harassed by a MIG-29 plane as well as surface-to-air missiles in two separate incidents on Monday.
It was the second successive harassment of Turkish planes reported by Turkey. The MIG-29 locked radar on the planes for four minutes and 30 seconds, while the missile systems threatened the planes for four minutes and 15 seconds.
Turkey reported on Monday that two Turkish jets were harassed by a MIG-29 on Sunday.
Russia's defence ministry rejected claims that its air strikes in Syria are targeting civilians or opposition forces.