Russia’s violation of Turkish airspace over the weekend “does not look like an accident”, Nato has said.
Secretary-General Jens Stoltenberg said Russia had not provided “any real explanation” of the violation, which had “lasted for a long time.”
Russia says a violation on Saturday lasted a few seconds and was due to poor weather. It says it is looking into claims of a second violation.
Russia’s latest strikes in Syria included targeting sites in Palmyra.
The strikes hit vehicles and weapons depots in and around the city, which is renowned for its ancient ruins and held by Islamic State militants, Syrian state TV reported on Tuesday.
Russia began its air campaign in Syria last Wednesday.
It says it is targeting Islamic State and other Islamists, but Nato and allied states have expressed concern that Russia is concentrating its attacks on other groups opposed to President Bashar al-Assad, some of them backed by the West.
Turkish reaction: Mark Lowen, BBC News, Istanbul
Turkey’s government has been enraged by these Russian incursions – and by Moscow’s military intervention in Syria as a whole.
First, any violation of Turkish airspace could lead to the object being shot down, which would dramatically escalate events. Second, there could be a mid-air collision close to Turkey’s borders, as this is the first time since World War Two that Russian and American combat planes have been in the skies over Syria. But third, Russia’s air strikes are the final nail in the coffin for Turkey’s “buffer zone” idea in northern Syria.
Ankara has continually pushed for this, ostensibly to allow some of the two million Syrians in Turkey to return – though critics say it’s designed to break up areas controlled by Syrian Kurds, who Turkey see as a threat. There was already opposition in the West to the plan. But Russia’s air strikes would make it almost impossible to implement.
Turkey has twice summoned the Russian ambassador – once over the first violation, which occurred on Saturday, and once over a second violation that Turkey says took place on Sunday.
Mr Stoltenberg called the Russian violation “unacceptable” and said Nato was taking it “very seriously”, though he could not confirm reports that a Russian warplane had locked its radar onto Turkish jets.
He added that Nato and Russia had military-to-military lines of communication open which had not been used, and that it would be natural to do so.
The head of Nato also said there had been “a substantial military build-up” by Russia in Syria, including ground troops and naval capabilities.
Russia’s deployment was of “great concern”, he said, calling on Moscow to avoid further escalating tensions with Nato.
Russian President Vladimir Putin has denied that civilians have been killed by Russian strikes in the past week, but evidence on the ground has indicated otherwise.
Syria’s conflict, which began in 2011, has left more than 250,000 dead and about half the country’s population displaced.
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By: BBC