It comes after Nato member Turkey complained that Russian jets had repeatedly violated its airspace.

Ministers are also expected to review a range of measures introduced in the wake of the Ukraine crisis.

BBC defence correspondent Jonathan Marcus says the alliance aims to make clear it will respond to any challenge.

Nato ministers are expected to express their solidarity with Turkey and also address increased concern among Baltic members following Russia's involvement in eastern Ukraine.

UK Defence Secretary Michael Fallon is expected to announce that Britain is ready to make a long-term troop deployment to the Baltic republics, our correspondent says.

But, he adds, ministers are meeting amid a deepening sense of crisis.

Russia's airspace violation 'no accident', says Nato

High-stakes gamble - Jonathan Marcus says the two incursions highlight the risk of a potential clash

What can Russia's air force achieve? Why should Russia be able to do any better than the US-led coalition?

Russia - an ally of embattled Syrian President Bashar al-Assad - began its air campaign a week ago, saying it was targeting Islamic State (IS) and other militant groups.

It has denied claims that its strikes have mainly hit non-IS targets.

On Wednesday, Russia said it had launched missile strikes against IS from warships in the Caspian Sea - about 1,500km (930 miles) away.

Russia's foreign ministry also said Moscow was willing to establish contact with the Free Syrian Army - a Western-backed rebel group - to discuss fighting IS "and other terrorist groups".

But US Defence Secretary Ash Carter - who will attend Thursday's Nato meeting - said coalition forces fighting IS in Syria would not co-operate with Russia.

"We believe Russia has the wrong strategy," he said. "They continue to hit targets that are not IS."

Airspace violations

Russia's air campaign in Syria has raised fears of accidental contact between Russian warplanes and those of the US-led coalition which have been targeting IS for the past year.

Pentagon officials revealed they recently had to carry out at least one "safe separation" manoeuvre to avoid a US jet coming too close to a Russian aircraft over Syria.

Nato Secretary-General Jens Stoltenberg said on Tuesday that Russia had not provided any real explanation for its violations of Turkish airspace, which "lasted for a long time" and "did not look like an accident".

He also pointed to reports of a build-up of ships and ground troops in Syria.

Russia's envoy to Ankara was summoned three times in response to recent incursions into Turkey's airspace.

Turkish President Recep Tayyip Erdogan warned the Kremlin that any "attack on Turkey means an attack on Nato".

"If Russia loses a friend like Turkey with whom it has a lot of co-operation it is going to lose a lot of things. It needs to know this," Mr Erdogan said.

The Syrian civil war began with an uprising against President Assad in 2011 that was brutally put down. It has since deteriorated into fighting between pro-government forces and various rebel groups - some of which also fight each other.

Amid the chaos, Islamic State militants have seized swathes of territory in Syria and Iraq.

(BBC)