NATO rejects Moscow's explanation that its warplanes violated the air space of alliance member Turkey by mistake.
NATO Secretary-General Jens Stoltenberg described the incidents as "extremely dangerous" and "unacceptable". He also expressed deep concern over reports of a substantial Russian military build-up, including ground troops and ships in the eastern Mediterranean.
At the same time, Turkish President Tayyip Erdogan said he was losing patience with Russian violations of his country's air space.
"An attack on Turkey means an attack on NATO," Erdogan warned at a Brussels news conference.
"I will not speculate on the motives ... but this does not look like an accident and we have seen two of them," .
The Russian Defence Ministry had said that an SU-30 warplane had entered Turkish air space along the border with Syria "for a few seconds" on Saturday, a mistake caused by bad weather. NATO says a plane also entered Turkish air space on Sunday, an incident Russia says it is looking into.
Stoltenberg declined to comment on whether the Russian planes had locked their radar on the F-16 Turkish jets scrambled on Saturday to remove Russian aircraft from the air space, usually a prelude to firing.