Regime troops backed by Lebanon's Hezbollah and Russian air strikes advanced Thursday in western Syria in a "vast offensive" against rebels, as NATO voiced alarm at Moscow's escalating military activity in the country.

Russia has dramatically stepped up its nine-day-old air war against foes of President Bashar al-Assad, with heavy bombing by warplanes and cruise missile strikes from the Caspian Sea.

A US official said four Russian missiles launched at Syria from warships Wednesday had crashed in Iran, but that was denied by Moscow.

A Syrian military source told AFP that regime forces had advanced in a key mountain range.

"They have seized most of the hilly region of Jeb al-Ahmar," which overlooks the strategic Sahl al-Ghab plain to the east and Assad's coastal stronghold of Latakia to the west, the source said.

The plain has been the focus of a months-long offensive by a rebel alliance including Al-Qaeda's Syrian affiliate, Al-Nusra Front.

The Syrian Observatory for Human Rights, a British-based monitoring group, confirmed that regime forces had advanced in the area.

Moscow says it is striking the Islamic State (IS) jihadist organisation and "other terrorists".

But Washington has accused Russia of targeting groups other than IS or Al-Nusra in more than 90 percent of its raids.

In Moscow, the defence ministry said Russia's air force hit 27 "terrorist" targets in central and northern Syria Wednesday night.

The ministry said it had destroyed eight IS strongholds near populated areas in Homs province, and hit 11 training camps linked to the group in Hama and Raqa provinces.

An anonymous US official said four of the missiles Russia launched on Syria from warships Wednesday instead crashed into in Iran, but did not provide an exact location.

But defence ministry spokesman General Igor Konashenkov said in Moscow that "any professional knows that during these operations we always fix the target before and after impact. All our cruise missiles hit their target."

There was no immediate reaction from Tehran.

- 'Consequences for Russia' -

Meanwhile, US Defence Secretary Ashton Carter forecast Thursday Russia would soon begin to suffer casualties of its own.

"This will have consequences for Russia itself which is rightly fearful of attacks... In coming days, the Russians will begin to suffer from casualties," Carter said at a NATO defence ministers meeting in Brussels.

NATO Secretary General Jens Stoltenberg said there had been a "troubling escalation" in Moscow's air campaign.

"We will assess the latest developments and their implications for the security of the alliance," he added.

"This is particularly relevant in view of the recent violations of NATO's airspace by Russian aircraft," Stoltenberg said.

Tensions between Russia and NATO member Turkey shot up this week after Russian aircraft infringed on Turkish airspace at least twice.

The Russian air war has provided cover for Assad's ground troops, who have lost swathes of the north, east and south of the country to jihadists and rebel groups since the conflict erupted in 2011.

The army appeared to regain ground Thursday, after chief of staff General Ali Abdullah Ayoub announced "a vast offensive to defeat the terrorist groups" and restore control over opposition-held areas.

Although Ayoub did not specify where the operation would take place, state TV said the army had targeted "terrorist positions" in the central province of Hama, killing 32 militants and destroying four armoured vehicles.

Russian and Syrian warplanes also conducted "precise strikes" on Al-Nusra positions in Latakia province, it said.

A military source in the Sahl al-Ghab plain told AFP Russian air strikes had targeted at least three villages there Thursday morning.

- Helicopter downed -

Backed by allied militia and Russian air cover, regime troops have retaken around a dozen villages in Hama, according to Syrian daily Al-Watan, which is close to the government.

At least 13 regime fighters and 11 rebels were killed, the Observatory said.

Rebel forces shot down a low-flying military helicopter, but it was unclear if it was Syrian or Russian, Observatory chief Rami Abdel Rahman said.

In Aleppo province, the monitor said a car bombing in the town of Hreitan killed 12 people and wounded a similar number.

Hreitan is controlled by a group of Islamist rebel factions including Al-Nusra.

Abdel Rahman said IS had been blamed for the bombing.

Since September 30, Russia has targeted areas it claims are controlled by IS and "other terrorists".

But non-jihadist rebels and their international backers insist that most of the areas targeted are not held by IS.

"Greater than 90 percent of the strikes that we've seen them take to date have not been against ISIL or Al-Qaeda-affiliated terrorists," said US State Department spokesman John Kirby, using an alternative acronym for IS.

One US-backed rebel faction has even accused Russian warplanes of destroying its arms depots and wounding several of its fighters.

burs/pg/al