DAMASCUS (AFP) - Syrian forces pressed their offensive against the Islamic State in Iraq and Syria (ISIS) on Monday (March 28), the day after seizing control of the ancient city of Palmyra in a major blow for the extremists.

Regime troops pushed on towards Deir Ezzor province, an ISIS bastion, the day after wresting control of the desert ruins with the help of Russian air strikes.

Analysts said the government's seizure of Palmyra was the biggest blow so far in the war against ISIS and a major coup both for Damascus and Moscow.

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

ISIS destroyed more than a dozen tombs and temples during its 10-month occupation of the Unesco World Heritage site, known as the "Pearl of the Desert".

Syria's antiquities chief said the monuments could be restored in five years, although a UN expert cast doubt on the time-frame.

Inside the city, army sappers worked to defuse bombs and mines planted by ISIS before they retreated on Monday. One soldier said more than 50 had been disarmed.

Outside, Syria's military turned their attention to other ISIS-held towns as they pushed towards Raqa, the group' de facto capital.

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

"That is the next goal for the Syrian army. They also have their eyes on Sukhnah," he added, referring to a town northeast of Palmyra.

The United States cautiously welcomed the victory for Assad, but said warned against allowing him to expand his "ability to tyrannise the Syrian people".

Concern has been mounting for the ancient city since ISIS overran it in May 2015 and began a campaign to destroy tombs and shrines it considers idolatrous.

In September, they demolished the 2,000-year-old Temple of Bel and a month later blew up the Arch of Triumph, from around 200 AD.

The extremists also used Palmyra's ancient theatre as a venue for public executions and murdered the city's 82-year-old former antiquities chief, Khaled al-Assaad.

Syria's head of antiquities, Maamoun Abdulkarim, told AFP that 80 per cent of the site was still "in good shape" and the ancient ruins could be restored in five years with Unesco's help.

But UN expert Annie Sartre-Fauriat said she was "very doubtful" that would be possible.

"Everyone is excited because Palmyra has been 'liberated', but we should not forget everything that has been destroyed," said Sartre-Fauriat, who belongs to a group of experts on Syrian heritage set up by Unesco in 2013.

Analysts said losing Palmyra was a major setback for ISIS, which has come under growing pressure from Syrian and Iraqi forces set on breaking apart its self-proclaimed "caliphate".

"The past week exemplifies the future of the Islamic State: relentless internal setbacks amid persistent external attacks," said the US-based Soufan Group.

Syria expert Thomas Pierret said the loss of Palmyra showed ISIS was "clearly weaker than in the past", but warned the extremists will likely fight harder to keep control of Raqa and Deir Ezzor.

US Secretary of State John Kerry vowed to pile more pressure on ISIS after meeting with Turkey's Foreign Minister Mevlut Cavusoglu in Washington.

But experts say Russia's role in Palmyra has also left the West scrambling to figure out Putin's game plan.

His announcement this month that Russia was withdrawing troops from Syria was greeted with much fanfare, but analysts said only 10-25 per cent of forces have left since then.

Moscow also openly admitted for the first time since it launched it operations in Syria last September that it has special forces on the ground as part of the offensive.

"All the talk in the West that Russia was going to ditch Assad was nonsense," said Pavel Felgengauer, a Russian military analyst.

"We are not planning to abandon him now. Russia wants Assad to stay in power and the goal is to give him a chance to win the civil war."

Assad's other key ally, Iran, has also hailed the recapture of Palmyra and pledged its continued financial and military assistance.

Hadi al-Bahra of the opposition National Coalition said the regime should have stopped ISIS from taking Palmyra to begin with.

"From the start, the regime's strategy was to allow the threat posed by Daesh to grow, in order to tell the West that either Daesh or Assad would prevail."