NATO is prepared to send troops to Turkey should it need to defend its ally as Russia intensifies its actions in Syria.

The security bloc's defence ministers met in Brussels on Thursday with Russia's intervention in Syria topping the agenda.

NATO's Secretary-General Jens Stoltenberg said as he arrived at the meeting: "NATO is ready to defend and protect all allies against any threat.

"We have seen increased turmoil, increased uncertainty, violence to the South of the NATO borders in Turkey, in Syria and Iraq bordering Turkey.

"And this is one of the reasons why we have increased readiness and preparedness of our forces... so we can be able to deploy forces if needed."

The discussions come after Moscow confirmed it had fired rockets at Islamic State targets in Syria from warships stationed in the Caspian Sea.

Twenty-six cruise missiles flew for 900 miles across Iran and Iraq before hitting plants for making shells and explosives, command centres, ammunition dumps and training camps, according to Russia's defence ministry.

But a US official said four of the Russian missiles crashed into Iran, which was rejected by a Russian defence spokesman.

"We do not talk with reference to anonymous sources," Major General Igor Konashenkov is quoted by the Interfax news agency as saying.

"We show the launch of our rockets and the targets they struck."

US Defence Secretary Ash Carter said the unpredictable actions of Russian forces were putting others at risk.

"We've seen increasingly unprofessional behaviour from Russian forces," Mr Carter said after the NATO talks ended.

"They violated Turkish airspace... They shot cruise missiles from the Caspian sea without warning."

Russia faced international criticism in recent days after its aircraft strayed into Turkish airspace. NATO dismissed Moscow's claims the incursions were accidental.

The former head of the British Army, Lord Dannatt, told Sky News that the West needs to rethink its strategy in Syria and ensure that Russia is part of those discussions.

"We're quite rightly concerned about Mr Putin's ... much more overt, aggressive involvement in the Syrian crisis," he said.

"But what we should be doing - I'm convinced we should be doing this - is setting up some sort of contact group with all interested parties.

"The key question to ask (is)... do we really want to get rid of president Assad? Or is defeating Islamic State our major objective?"

Meanwhile, Syria's military chief-of-staff Lt Gen Ali Abdullah Ayoub said that his forces were launching a "big attack" to "liberate areas and towns which have suffered from terrorism".

TV pictures from the state broadcaster showed large scale artillery and rocket launchers firing in what the station said was the Hama and Idlib regions.

Russian defence ministry said its air force hit 27 Islamic State targets overnight in the Syrian provinces of Homs, Hama and Raqqa.

Activists said troops working for the government pushed into the Ghab plain and rural parts of Latakia province on Thursday.

Britain-based Syrian Observatory for Human Rights said Wednesday's Russian airstrikes in Idlib killed at least seven civilians.

The Observatory said Syrian rebels claimed to have shot down a helicopter in Hama province on Thursday but it was not known if it was Russian or Syrian.