Syrian troops on Sunday seized the key Islamic State group bastion of Al-Qaryatain, dealing the jihadists a new blow in the country's centre a week after expelling them from Palmyra, state television said.

The Syrian Observatory for Human Rights monitoring group however said fighting was still raging in the east and southeast of the town, which is located in the desert in Homs province.

"The army with backing from supporting forces (pro-regime militia) brings back complete security and stability to the town of Al-Qaryatain, after crushing Daesh terrorists' last remaining positions there," state television said, using an Arabic acronym for IS.

But the Britain-based Observatory said the army was still fighting the jihadists in the town.

"Clashes are still ongoing in the east and southeast of the town," it said.

The advance came after the Russian-backed Syrian army dealt IS a major blow on March 27 by seizing the ancient city of Palmyra, known as the "Pearl of the desert", from the jihadists.

Al-Qaryatain is located some 120 kilometres (75 miles) southwest of Palmyra.

Its recapture will allow the army to secure its grip over the ancient city, where jihadists destroyed ancient temples and executed around 280 people during their 10-month rule.

Once Al-Qaryatain returns to government control, "of the whole of Homs province, IS will only hold its bastion in Sukhna" northeast of Palmyra, Observatory chief Rami Abdel Rahman said.

"The recapture of Al-Qaryatain will also allow the army to reclaim the whole of the Syrian desert" spreading all the way south to the Iraqi border, Abdel Rahman added.

A ceasefire brokered by the United States and Russia but which does not apply to the fight against jihadists has enabled the Syrian army to focus its efforts on IS.

- IS executions -

The group has also lost a string of high-ranking commanders in recent weeks to strikes by the US-led coalition which launched an air campaign against the jihadists in Iraq and Syria in 2014.

A drone strike on Wednesday, likely by the coalition, killed Abu al-Haija, a Tunisian commander summoned by IS leader Abu Bakr al-Baghdadi from Iraq.

Fifteen IS commanders accused of revealing Abu al-Haija's position have since been executed by the jihadist group, the Observatory said Sunday.

The fate of another 20 men accused of collaborating with the US-led coalition remains unknown, it added.

"This is the highest number of executions of security officials by IS," Abdel Rahman said.

The Observatory said on Sunday that 12 fighters from Lebanon's Shiite Hezbollah were killed fighting the Al-Qaeda-affiliated Al-Nusra Front and allied rebels in the northern province of Aleppo.

They died "in shelling and fighting in the south of Aleppo province, during the fierce offensive by Al-Nusra... and rebels the day before yesterday (Friday)," the group said.

Hezbollah has since 2013 been openly fighting in Syria in support of President Bashar al-Assad's regime.