Four Russian cruise missiles aimed at targets in Syria crashed in Iran, a US official said Thursday, as regime troops backed by Lebanon's Hezbollah pressed a "vast offensive" against rebels in the war-torn country's west.

The missiles were thought to be among a salvo fired Wednesday from Russian warships in the Caspian as part of a nine-day-old air war targeting foes of President Bashar al-Assad.

Russia hit back at the claims, saying all the shots were on target, and the defence ministry posted a graphic on its website showing 26 missiles flying over Iran and Iraq before striking inside Syria. Tehran made no comment.

"Any professional knows that during these operations we always fix the target before and after impact. All our cruise missiles hit their target," ministry spokesman General Igor Konashenkov said in a statement.

Moscow launched an air war in Syria at the end of last month it said was aimed at the Islamic State group and other "terrorist" organisations fighting in the country's four-year-old civil war.

Russia's air force hit 27 "terrorist" targets in central and northern Syria Wednesday night, the defence ministry said, including eight IS strongholds in Homs province and 11 training camps in Hama and Raqa provinces.

Western powers have dismissed these claims as window-dressing for a campaign that primarily seeks to prop up Assad's embattled regime against a much broader group of rebels.

Washington said more than 90 percent of Russian raids have targeted groups other than IS or Al-Qaeda's Syrian affiliate, Al-Nusra Front.

Another US official said the missiles that landed in Iran were Kalibr-NK cruise missiles, which Russia had "used for the first time in a combat setting".

"They appeared to help operations by Iranian-backed Hezbollah" in Syria, where the powerful Lebanese Shiite group has been fighting alongside regime forces, the official added.

- 'Troubling escalation' -

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.

A Syrian military source said Thursday the Russian strikes had helped regime forces take back territory in an area that has been the focus of a months-long offensive by a rebel alliance, including Al-Nusra.

"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 western coastal stronghold of Latakia, the source said.

The Syrian Observatory for Human Rights, a British-based monitoring group, confirmed the advance.

Tensions have been rising between Russia and Western powers over the air campaign, which has seen Russian jets violate the airspace of NATO member Turkey at least twice.

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 and pledged to "assess the latest developments and their implications for the security of the alliance".

- 'Serves the Christians' -

The Melkite Greek Catholic archbishop of Aleppo Jean-Clement Jeanbart on Thursday welcomed Moscow's intervention, which he told Swiss television "serves the Christians' cause".

Syria's 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.

State TV said the army had targeted "terrorist positions" in the central province of Hama, killing 32 militants and destroying four armoured vehicles, while Russian and Syrian warplanes also hit Al-Nusra positions in Latakia province.

A military source in the Sahl al-Ghab plain told AFP Russia 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, 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 blamed on IS in the town of Hreitan -- controlled by a group of Islamist rebel factions including Al-Nusra -- killed 12 people and wounded a similar number.

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