Syrian troops and allied militia backed by a fresh wave of Russian air strikes and cruise missiles fired from warships attacked rebel forces on Thursday as the government extended an offensive to recapture territory in the west of the country.

The assault focused on western areas where rebel advances earlier this year had threatened the coastal region vital to President Bashar al-Assad's support base.

The Russian Defence Ministry said it fired missiles from ships in the Caspian Sea for a second day and had hit weapons factories, arms dumps, command centres and training camps.

US officials said they believed four Russian cruise missiles bound for Syria had crashed en route in Iran. Russia's Defence Ministry insisted the missiles had reached their targets in Syria.

The White House declined to comment and State Department spokesman John Kirby said he could not confirm the missiles had crashed, while adding that the report pointed towards the need for procedures to prevent clashes with US planes targeting Islamic State militants in Syria.

Since Russia began air strikes last week it has described all its targets as belonging to the Islamic State group, although most have been in areas controlled by other rebel movements where Islamic State has little or no presence.

On Thursday, Moscow said its air force hit 27 Islamic State targets in the provinces of Homs, Hama and Raqqa, while Kirby said Secretary of State John Kerry had expressed his concern to Russian Foreign Minister Sergei Lavrov by telephone that most of the targets hit so far were not related to Islamic State.

On the ground, forces loyal to the Syrian government targeted insurgents in the Ghab Plain area in the west of the country, with heavy barrages of surface-to-surface missiles as Russian warplanes bombed from above, according to the British-based Syrian Observatory for Human Rights and a rebel there.

"There is an attempt by the regime to advance but the situation is under our control," said a fighter in the area from the Ajnad al-Sham insurgent group who uses the name Abu al-Baraa al-Hamawi.

Speaking by Internet link, he told Reuters Russian jets had been bombing since dawn, their most ferocious attack on the area so far. He said rebels had managed to destroy a number of Syrian tanks in a counter attack on Wednesday.

"God willing we will repeat the massacre of the north Hama countryside as happened yesterday," he said, referring to that counter attack. "We have faced more violent attacks than this in the past."

The Observatory said rebels had shot down a helicopter in Hama province in western Syria. It was unclear if it was Syrian or Russian.

Syria said it had set in train a major military operation to regain the upper hand on the battlefield. Its civil war began more than four years ago and has now killed 250,000 people, sent millions into exile as refugees and drawn in world and regional powers.

Assad's armed forces "have launched wide-ranging attacks to deal with the terrorist groups, and to liberate the areas which had suffered from the terrorist rule and crimes", Syria's army Chief of Staff, Lieutenant General Ali Abdullah Ayoub, was quoted as saying by state media.

New fighting units

Ayoub did not say which areas were being targeted. He said new fighting units, including one called the Fourth Assault Corps, had been set up to wage the campaign and the army now held the military initiative.

Sources in the region say Iran has sent hundreds of troops to back Syrian forces in a ground campaign coordinated with Russia's air assault. Assad's government also relies on support from Hezbollah, the Shi'ite militia from neighbouring Lebanon.

The Observatory's head, Rami Abdulrahman, said an assault launched by the army and its foreign allies on Wednesday in nearby areas of Hama province had so far failed to make significant gains, however.

"At least 13 regime forces were killed...The clashes also killed 11 (rebel) fighters," he said in a statement, and the numbers were expected to rise as more casualties were confirmed.

Around 15 army tanks and armoured vehicles had been destroyed or immobilised by rebel missile strikes, Abdulrahman said.

The operation that began on Wednesday in Hama appears to be the first major assault coordinated between Syrian troops and militia on the ground with Russian warplanes and naval ships.

Hama province's Ghab Plain lies next to a mountain range that forms the heartland of Assad's Alawite sect.

Recapturing it from the alliance of rebel groups which includes al Qaeda's Nusra Front and which thrust into the area in late July, would help secure Assad's coastal heartlands and could provide a platform to drive rebels back from other areas.

Anger over air space

The United States has been leading a separate air campaign against Islamic State targets for a year, and the arrival of Russian war planes last week means the Cold War superpower foes are now flying combat missions in the same air space for the first time since World War Two.

Washington and its allies want Assad to leave power and accuse Moscow of using a campaign against Islamic State as a pretext to target Assad's other enemies, many of which receive help from countries that oppose him. Russia, allied to Damascus since the Cold War, says Assad's government should be part of an international campaign against extremists.

Neighbouring Turkey, a NATO member, has been angered by violations of its air space by Russian warplanes and the Western alliance said it was prepared to send troops to Turkey to defend it.

"NATO is ready and able to defend all allies, including Turkey against any threats," NATO Secretary-General Jens Stoltenberg told reporters as he arrived for a meeting in Brussels of the alliance's defence ministers.

At the meeting, Turkey appealed to its NATO allies to shore up missile defences.