BEIRUT - Regime forces advanced Thursday in western Syria in a “vast offensive” against armed opposition groups, as NATO voiced alarm at escalating Russian military activity in the war-torn country.

Moscow 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 Syrian military source told AFP on Thursday that the army, backed by Russian raids and allied militia including the powerful Lebanese group Hezbollah, 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 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. US Defence Secretary Ashton Carter said Thursday Russia would soon begin to suffer casualties.

“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 said it was prepared to send troops to Turkey to defend its ally after violations of Turkish airspace by Russian jets bombing Syria and Britain scolded Moscow for escalating a civil war that has already killed 250,000 people.

Officials at the US-led alliance are still smarting from Russia’s weekend incursions into Turkey’s airspace near northern Syria and Nato defence ministers are meeting in Brussels with the agenda likely to be dominated by the Syria crisis.

“Nato is ready and able to defend all allies, including Turkey against any threats,” Nato’s Secretary-General Jens Stoltenberg told reporters as he arrived for the meeting.

“Nato has already responded by increasing our capacity, our ability, our preparedness to deploy forces including to the south, including in Turkey,” he said, noting that Russia’s air and cruise missile strikes were “reasons for concern”.

Turkish President Recep Tayyip Erdogan on Thursday warned Russia risked losing a deal to build his country’s first nuclear power plant and its status as its main gas supplier, as the diplomatic row intensified.

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 on Thursday, after its 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, Syrian state TV reported that 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 positions held by Al-Nusra Front in Latakia province, it said.

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

Backed by allied militia and Russian air cover, regime troops have retaken around a dozen villages in Hama province, the Syrian daily Al-Watan, which is close to the government, reported. 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 the aircraft was Syrian or Russian, Observatory chief Rami Abdel Rahman said. He could not confirm what had happened to those inside.

Since September 30, the Russian air force has been conducting strikes on Syrian targets that Moscow says are held 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.

“They’ve been largely against opposition groups that want a better future for Syria and don’t want to see the Assad regime stay in power.”

One US-backed rebel faction, Suqur al-Jabal (Falcons of the Mountain), has accused Russian warplanes of destroying its arms depots and wounding several of its fighters.