NATO on Tuesday rejected Moscow's explanation that its warplanes had violated alliance member Turkey's air space by mistake and said Russia was sending more ground troops to Syria and building up its naval presence.
With Russia extending its air strikes to include the ancient Syrian 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 at the weekend. 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.
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.
A U.S. official told Reuters the incursions had lasted more than a few seconds and described Moscow's assertion that they were an accident as "far-fetched".
The Turkish military said a further incident took place on Monday when a MIG-29 fighter and Syria-based missile systems "interfered" with eight Turkish F-16 jets patrolling along the Syrian border.
The MIG-29 locked its radar onto the Turkish patrol for 4 minutes 30 seconds, and a Syria-based missile system locked onto them for 4 minutes 15 seconds, the Turkish military said.Russia flies MIG-29s, as does Syria's own air force.
Stoltenberg said the U.S.-led alliance had not received "any real explanation" from Russiaabout the incursions.
Air space violations
Disagreement over the air space violations comes as Russia and the west dispute the aims of Moscow's air campaign. Moscow says it is attacking Islamic State, but most of its air strikes have hit territory held by other rebel groups fighting against its Syrian ally, President Bashar al-Assad.
Western countries, Arab states and Turkey, who are waging their own bombing campaign against Islamic State but also want Assad to leave power, say Moscow is using Islamic State as a pretext to target Assad's other foes. Russia says the Assad government should be the centrepiece of international efforts to combat extremism.
With Russian and NATO planes now flying combat missions in the same air space, a danger is that the Cold War enemies could fire on each other.
The skies in the area have seen incidents in the past. A Turkish jet was shot down by the Syrian air force in 2012 over the Mediterranean and earlier this year Turkey shot down a Syrian helicopter that Ankara said violated its airspace.