Nato said it was prepared to send troops to Turkey to defend its ally after violations of Turkish airspace by Russian jets bombing Syria.
Britain scolded Moscow for escalating the civil war in the Middle East country that has already killed 250,000 people.
Officials at the US-led alliance are still smarting from Russia’s weekend incursions into Turkey’s airspace near northern Syria and NATO defence ministers are meeting in Brussels with the agenda likely to be dominated by the Syria crisis.
“Nato is ready and able to defend all allies, including Turkey against any threats,” Nato’s secretary-general Jens Stoltenberg told reporters as he arrived for the meeting.
“Nato has already responded by increasing our capacity, our ability, our preparedness to deploy forces including in Turkey,” he said, noting that Russia’s air and cruise missile strikes were “reasons for concern”.
As Russian and US planes fly combat missions over the same country for the first time since World War 2, NATO is eager to avoid any international escalation of the Syrian conflict that has unexpectedly turned the alliance’s attention away from Ukraine following Russia’s annexation of Crimea last year.
The incursions of two Russian fighters in Turkish airspace on Saturday and Sunday has brought the Syria conflict right up to Nato’s borders, testing the alliance’s ability to deter a newly assertive Russia without seeking direct confrontation.
While the United States has ruled out military cooperation with Russia in Syria, Nato defence ministers will discuss how to encourage Russia to help resolve the crisis, betting that Moscow also wants to avoid being bogged down in a long conflict.
For 40 years, Nato’s central task was deterring Russia in the east during the Cold War, but now, after a decade-long involvement in Afghanistan, the alliance is facing a reality-check close to home, with multiple threats near its borders.
Divisions between eastern Nato members, who want to keep the focus on the Ukraine crisis, and others who fret about IS militants, risk hampering a unified response from the 28-nation North Atlantic alliance.