NATO Secretary General Jens Stoltenberg condemned Russian warplanes' incursions into Turkey's airspace calling it "unacceptable and dangerous'' as the incidents only added tensions to the ongoing Russia's military intervention in Syria.

Stoltenberg confirmed the second incursion by Russian warplanes into Turkish airspace on Tuesday. The warplanes were apparently providing support to Russian troops already on the ground in Syria, CNN reported.

"We see the violation of the airspace becomes more important, more dangerous, because it happens in a context where we see more fighting, more Russian military presence in Syria," said Stoltenberg.

Turkish President Tayyip Erdogan reportedly expressed dismay and was "losing patience" with Russian violations of his country air space.

"An attack on Turkey means an attack on NATO,'' Erdogan warned at a Brussels news conference, according to Reuters.

In addition to the incursion reports, the Turkish air force, according to CNN, has reported that a MiG-29 fighter jet of an unidentified nationality had interfered Monday with the flight of eight Turkish F-16s on the Turkey-Syria border. The Turkish air force said the unidentified jet put its planes under a radar lock for four minutes and 30 seconds.

"We also have seen two of them, two violations of Turkish airspace," Erdogan said at a news conference, CNN reported. "Intelligence that we have received provides me with reason to say it doesn't look like an accident."

Stoltenberg earlier expressed doubt that Russia, which has been conducting airstrikes in Syria, is interested primarily in fighting the Islamic State terrorist group. "I'm also concerned that Russia is not targeting [ISIS] but instead attacking the Syrian opposition and civilians.''

He also expressed alarm at how the Russian military presence is growing on several fronts in Syria, CNN said.

"So we have seen a substantial military build-up by Russia in Syria, both in the air with the combat planes and air defence systems, but also an increasing number of ground troops," he said. "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."

Moscow reportedly admitted making a mistake and apologised after its warplanes violated Turkey's airspace.

Ankara has accepted the apology, saying "there is no ill feeling between the two countries,'' according to Reuters.

Turkey has been a member of NATO since 1952. Alliance rules state that an attack on any of the 28 member states should be treated as an attack on all alliance members, CNN said.