NATO Secretary General Jens Stoltenberg said Thursday there had been a "troubling escalation" in Russian military activity in Syria, after Moscow dramatically expanded its air campaign.
Stoltenberg also said that NATO was ready to deploy forces in key alliance member Turkey if needed after Ankara complained about a series of airspace violations by Russian planes.
"In Syria, we have seen a troubling escalation of Russian military activities," Stoltenberg said as he went into a NATO defence ministers meeting dominated by the Syrian crisis.
"We will assess the latest developments and their implications for the security of the alliance," he said.
"This is particularly relevant in view of the recent violations of NATO's airspace by Russian aircraft," he added.
Russia launched its air campaign against what it terms "terrorist" targets in Syria last week but in the process, it has infringed on key NATO ally Turkey's airspace, prompting strong protests from the US-led alliance and Ankara.
On Wednesday, Moscow upped the ante dramatically with the launch of an unprecedented series of cruise missile attacks to cover a Syrian army ground offensive against rebels seeking the ouster of long-term Russian ally President Bashar al-Assad.
NATO has stationed anti-missile Patriot batteries in Turkey to protect it from any spillover of the Syrian conflict but they are due to be removed later this year.
- 'NATO able to defend Turkey' -
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Asked if NATO would now consider extending their mission in light of Russia's actions, Stoltenberg said: "NATO is ready and able ready to defend all allies against any threat, including Turkey."
He said NATO's decision to boost its readiness was not driven solely by Russia's intervention in favour of pro-Moscow rebels in Ukraine.
"It is a response both to the challenges we see to the East but also to the challenges we see to the South," he said.
"NATO has already responded by increasing our capacity, our ability our preparedness to deploy forces, including to the South, including in Turkey, if needed," he added.
Stoltenberg cautioned that the situation demanded more than ever a political solution and urged Russia to play a "constructive" role by not targetting the rebels in support of Assad.
"What we see is that there is a renewed need for political initiatives ... because in the long term there is no military solution," he said.
The 28-nation alliance has changed tack radically in the fallout from the Ukraine crisis after years of defence cuts, with leaders agreeing last year to increase spending and to set up a very rapid response force which should be operational from next year.
The makeover is meant to ensure that NATO can respond within days rather than weeks and months to any crisis in a fast changing world where the threat can take many forms, from a conventional military attack to cyber warfare.
The defence ministers will also discuss the situation in Afghanistan where the Taliban's recent capture of the important city of Kunduz has badly shaken confidence in the government's ability to hold the rebels off despite NATO support.