Government forces advanced 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.
Government forces appeared to have regained 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 fighters and destroying four armoured vehicles.
At least 13 government fighters and 11 opposition fighters were killed in the clashes, the UK-based Syrian Observatory for Human Rights said.
Rebels shot down a low-flying military helicopter, but it was unclear if the aircraft was Syrian or Russian, Rami Abdelrahman, the director of the Syrian Observatory, said. He could not confirm what had happened to those inside.
Elsewhere in Syria, backed by Russian air strikes, government forces fought heavy battles with armed opposition groups for control of a hilltop in an mountainous range close to President Bashar al-Assad's coastal heartland.
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The fighting was centred around Jeb al-Ahmar, a highland area in Latakia province which if captured would allow the army to more effectively pound rebel positions in the nearby Ghab Plain, said Abdelrahman.
A Syrian military source told AFP news agency on Thursday that the army, backed by Russian raids and allied groups including the Lebanese group Hezbollah, had advanced into the 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-Nusra Front.
Russia has dramatically stepped up its nine-day-old air war against foes of Assad, with heavy bombing by warplanes and cruise missile strikes from the Caspian Sea.
Russia says it is striking the Islamic State of Iraq and the Levant (ISIL) group and "other terrorists".
The US has accused Russia of targeting groups other than ISIL or al-Nusra in more than 90 percent of its raids.
'Consequences for Russia'
US Defence Secretary Ashton Carter said on 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 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.
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.