NATO on Tuesday rejected Moscow's explanation that its warplanes violated the air space of alliance member Turkey at the weekend by mistake and said Russia was sending more ground troops to Syria.
With Russia extending its air strikes to include the ancient city of Palmyra, 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.
NATO Secretary-General Jens Stoltenberg said the alliance had reports of a substantial Russian military build-up in Syria, including ground troops and ships in the eastern Mediterranean.
"I will not speculate on the motives ... but this does not look like an accident and we have seen two of them," Stoltenberg said of the air incursions over Turkey's border with Syria. He noted that they "lasted for a long time".
The incidents, which NATO has described as "extremely dangerous" and "unacceptable", underscore the risks of a further escalation of the Syrian civil war, as Russian and U.S. warplanes fly combat missions over the same country for the first time since World War Two.