Nato's secretary-general said on 6 October he doubted Russia's explanation that its weekend violations of Turkey's airspace were a mistake because there were two such incursions and they lasted longer than just a few seconds.

Speaking at a news conference, Jens Stoltenberg said it remained unclear what the purpose of the incursions were despite Russia's claim that the first incursion was "accidental" and was to blame on "unfavourable weather conditions in the area".

"I will not speculate on the motives. I will just reiterate or restate that this is a serious violation of Turkish airspace. It should not happen again," Stoltenberg said.

He added the US-led Nato alliance, of which Turkey is a member, had not received "any real explanation" of what happened. He said he had not had any direct contact with Moscow but Nato has discussed the possibility of using its military lines of communication with Russia. He warned that new violation of Turkish airspace could have serious consequences.

"It's unacceptable to violate the airspace of another country and this is exactly what we were afraid of. That incident may create dangerous situations and therefore it is also important to make sure that this does not happen again. That's also the reason why we send this very clear signal," Stoltenberg said.

The Russian government said earlier it is looking into a new allegation that one of its jets operating in Syria violated Turkish airspace for a second time. Turkey complained late on 5 October that a Russian warplane had violated its airspace on 4 October, the second such breach in three days, prompting Ankara to once again summon Moscow's ambassador.

The first such incursion, on 3 October, prompted the US and Nato to denounce Russia, and Ankara to threaten to respond, raising the prospect of direct confrontation between the former Cold War adversaries.

Moscow said the Syrian airbase from which Russian planes were flying missions, Khmeimim, was located about 30km from the Turkish border and that its aircraft had to approach it from the north in certain weather conditions. Russia has denied another Turkish assertion that one of its planes locked its radar onto two Turkish fighter jets.