Syrian troops and militia backed by Russian warplanes mounted what appeared to be their first major coordinated assault on Syrian insurgents on Wednesday and Moscow said its warships fired a barrage of missiles at them from the Caspian Sea, a sign of its new military reach.

The combined operation hit towns close to the main north-south highway that runs through major cities in the mainly government-held west of Syria, said the Syrian Observatory for Human Rights, a British-based group which tracks the conflict via a network of sources within the country.

Ground attacks by Syrian government forces and their militia allies using heavy surface-to-surface missile bombardments hit at least four insurgent positions and there were heavy clashes, the head of the Observatory, Rami Abdulrahman, said.

The Iranian-backed Hezbollah militia took part in the fighting, according to a regional source who is familiar with the military situation in Syria.

Abdulrahman said later there was no sign that Syrian troops and their allies had made any tangible advances on the ground.

They briefly entered one town, but were forced to pull back, he said, and around 15 of their tanks or armoured vehicles had been either destroyed or disabled.

Islamic State militants have seized much of Syria since civil war grew out of anti-government protests in 2011, but the areas targeted in Wednesday's combined assault are held by other rebels, some US-backed, fuelling accusations by Russia's critics that its real aim is to help the government.

Moscow says it shares the West's aim of preventing the spread of Islamic State, and Russian Defence Minister Sergei Shoigu told President Vladimir Putin during a televised meeting that four Russian warships in the Caspian Sea had launched 26 missiles at Islamic State in Syria earlier in the day.

The missiles would have passed over Iran and Iraq to reach their targets, covering what Shoigu described as a distance of almost 1,500 km (900 miles), the latest display of Russian military power at a time when relations with the West are at a post-Cold War low over Ukraine.

The terrain-hugging Kalibr cruise missiles, known by NATO by the codename Sizzler, fly at an altitude of 50 metres and are accurate to within three metres, the Russian defence ministry said.

The air campaign in Syria has caught Washington and its allies on the back foot and alarmed Syria's northern neighbour Turkey, which says its air space has been repeatedly violated by Russian jets.

Ankara summoned Russia's ambassador for the third time in four days over the reported violations, which NATO has said appeared to be deliberate and were "extremely dangerous".

Turkey said Syria-based missile systems harassed its warplanes on Tuesday while eight F-16 jets were on a patrol flight along the Syria border.

Iraq looks to Russia

Syrian state television quoted a military source as saying the missiles fired by Russian ships targeted 11 Islamic State positions in Raqqa, Aleppo and Idlib.

The missiles destroyed bomb-making factories, command posts, weapons and ammunition and fuel depots, as well as "terrorist training centres", the TV said.

Russian air strikes destroyed the main weapons depots of a US-trained rebel group, the Liwa Suqour al-Jabal, their commander said.

In conversation with Shoigu, Putin said it was too early to talk about the results of Russia's operations in Syria and ordered his minister to continue cooperation with the United States, Turkey, Saudi Arabia, Iran and Iraq on the crisis.

US Defense Secretary Ash Carter said the United States would not cooperate militarily with Russia in Syria, although it was willing to hold discussions to secure the safety of its own pilots bombing IS targets in Syria.

The Pentagon said US-led coalition aircraft bombing Islamist militants in Syria had been re-routed at least once in the last six days to avoid a close encounter with Russian planes.

Turkish Prime Minister Ahmet Davutoglu said only two of 57 Russian air strikes in Syria so far had hit Islamic State, while the rest had been against the moderate opposition, the only forces fighting the hard-line insurgents in northwestern Syria.

But in Iraq, the head of parliament's defence and security committee said Baghdad may request Russian air strikes against Islamic State on its soil soon and wants Moscow to have a bigger role than Washington in fighting the group.

Iraq's government and powerful Iranian-backed Shi'ite militias question the United States' resolve in fighting Islamic State militants, who control a third of the country, saying US-led coalition air strikes are ineffective.

The Kremlin said it had not received any official request from Iraq for air strikes against Islamic State there.

Air support only so far

Russia's military build-up in Syria included a growing naval presence, long-range rockets and a battalion of troops backed by Moscow's most modern tanks, the US ambassador to NATO said.

"There is a considerable and growing Russia naval presence in the eastern Mediterranean, more than 10 ships now, which is a bit out of the ordinary," Douglas Lute told reporters ahead of a meeting of alliance defence ministers in Brussels.

Abdulrahman said Russia appeared to have stuck to air support on Wednesday. The assault followed a report by Reuters last week that allies of Syrian President Bashar al-Assad, including Iranians, were preparing to recapture territory lost by the government to rebels in rapid advances this year.

Hezbollah-run al-Manar television said in a newsflash that "an operation by the Syrian army started in a number of villages and towns in the northern countryside of Hama province".

A video posted by the media office of an opposition group in Hama province on YouTube purported to show heavy rocket strikes by pro-government forces on Wednesday hitting an area in the northern Hama countryside.

Other footage from Hama showed rebels from the Free Syrian Army firing anti-tank missiles and hitting two army tanks.