Nato is ready to defend Turkey - including sending in troops "if needed", Nato chief Jens Stoltenberg has said.
The head of the US-led alliance warned it would be prepared to protect its allies from "threats", following reports of Russian jets in Turkish airspace.
Moscow has claimed that its military incursion, which came as it launched missiles at Isis targets in Syria, was "not intentional or important", but Mr Stoltenberg said the incidents were "very serious".
The secretary-general also told reporters before a meeting of Nato defence ministers in Brussels on Thursday that the alliance had begun preparing its forces for possible deployment.
"Nato is ready and able to defend all allies, including Turkey against any threats," Mr Stoltenberg said.
"Nato has already responded by increasing our capacity, our ability, our preparedness to deploy forces including to the south, including in Turkey."
He added that Russia's air and cruise missile strikes were "reasons for concern", Reuters reported, and warned that “incidents, accidents, may create dangerous situations.”
“Therefore it is also important to make sure that this doesn’t happen again.”
The meeting in Brussels is expected to be dominated by discussion of the crisis in Syria.
The Russian Defence Minister Sergei Shoigu said on Russian television that 26 missile strikes were launched on Wednesday from four warships in the Caspian sea, destroying all 11 intended targets.
But Turkish Prime Minister Ahmet Davutoğlu told the Daily Sabah only two out of 57 Russian air strikes in Syria hit Isis targets.
The rest, he said, targeted ‘moderate opposition groups’.
