Syrian forces strengthened their hold on Palmyra on Monday and pushed forward against the Islamic State group after dealing it a major blow by retaking the ancient city.

The country's antiquities chief said the treasured monuments damaged by the jihadists could be restored in five years, but a UN expert cast doubt on the estimate.

Backed by Russia, pro-government fighters overran Palmyra on Sunday morning after nearly 10 months of IS rule.

President Bashar al-Assad hailed the victory as "fresh proof of the efficiency of the Syrian army and its allies in fighting terrorism".

Regime forces turned to nearby IS-held towns.

"The army was concentrated around Al-Qaryatain, and today the military operations began there," a military source in Palmyra said on Monday.

"That is the next goal for the Syrian army. They also have their eyes on Sukhnah" towards the northeast, the source told AFP.

The Syrian Observatory for Human Rights said Sukhnah's capture would bring government forces to the gates of oil-rich Deir Ezzor province, an IS bastion.

IS overran Palmyra - a Unesco world heritage site known as the "Pearl of the Desert" - in May 2015 and used its ancient theatre for public executions as the world watched in horror.

The extremist group blew up temples and tower tombs, as part of it campaign against pre-Islamic monuments it considers blasphemous.

An AFP correspondent in Palmyra saw the Temple of Bel and the Arch of Triumph in pieces on Sunday, with some large stones marred by spray-painted messages in support of IS.

'Five years' to restore  

On Monday, army sappers were still working to defuse roadside bombs and mines that IS had planted before it retreated.

One soldier said more than 50 bombs had already been disarmed.

Antiquities chief Maamoun Abdulkarim said that with Unesco support, his department would need five years to restore the monuments destroyed by IS, and that a preliminary assessment showed 80% of the ancient site was "in good shape".

The famed Lion of Al-Lat, shattered by IS last year, could be put back together and there was not the widespread looting that had been feared, he said.

"We need to start with the damage in the citadel immediately, because it can't afford all the damage that it has suffered."

A Paris-based UN expert, however, said she was "very doubtful" that would be possible.

"When I hear that we are going to reconstruct the temple of Bel, that seems illusory," said Annie Sartre-Fauriat, who belongs to a group of experts on Syrian heritage set up by Unesco in 2013.

The US-based Soufan Group said IS "suffered an undeniable loss" with its defeat in Palmyra.

The jihadist organisation has come under growing pressure from Syrian and Iraqi forces set on breaking apart its self-proclaimed "caliphate".

The group has been responsible for a spate of deadly attacks abroad, most recently in Brussels, where 35 people were killed last week.  

"The past week exemplifies the future of the Islamic State: relentless internal setbacks amid persistent external attacks," the Soufan Group said in a briefing paper.

Tougher battles ahead?  

Syria expert Thomas Pierret said the loss of Palmyra showed IS was "clearly weaker than in the past".

But he said he expected the group to "fight much harder" to maintain control of its de facto capital of Raqa and Deir Ezzor to the east.

"After all, Palmyra was a forward post."

Long keen to portray his army as the vanguard of the fight against the jihadists, Assad hailed Sunday's victory in Palmyra as an "important achievement".

Russian President Vladimir Putin, whose government provided air and ground support for the offensive, congratulated Damascus on its success.

In a statement carried by state news agencies, Russia's armed forces said it would dispatch deminers to Palmyra within days.

Assad's other key ally, Iran, has hailed the victory and pledged its continued financial and military assistance.

Iran's president, Hassan Rouhani, spoke with Putin by phone, according to a tweet posted on Monday.

He "stressed the need for a lasting ceasefire in #Syria & acceleration of negotiations", the tweet read, referring to a UN-brokered peace process aimed at ending Syria's five-year war.

Back on the ground, a government fighter in Palmyra told AFP that troops would now aim to "find out what happened to the families that were in Palmyra... We're afraid they've been kidnapped."

Some 15,000 of Palmyra's 70,000 residents had stayed on under IS rule.

The residential area of the city saw heavy fighting between government forces and IS.

Most apartment blocks bore marks of the fighting and several had completely collapsed.