Syrian government forces, backed by Russian air power, have recaptured the ancient city of Palmyra from the Islamic State of Iraq and the Levant (ISIL) after days of intense fighting, state media and a monitoring group said.

Syrian state television quoted a military source as saying the army and allied militias had taken "complete control over the city of Palmyra".

The Britain-based Syrian Observatory for Human Rights said gunfire could still be heard in the eastern part of the city on Sunday morning, but that the bulk of ISIL's forces had retreated.

The group's director, Rami Abdulrahman, said that 400 ISIL fighters died in the battle, which he described as the biggest single defeat for the group since it announced its self-declared caliphate in Iraq and Syria nearly two years ago.

SANA quoted Syrian President Bashar al-Assad as saying: "This is an important achievement and new evidence of the effectiveness of the strategy followed by the Syrian army and its allies in the war against terrorism".

READ MORE: Syrian regime seeks symbolic victory in Palmyra

"The effectiveness of this strategy is further highlighted especially as opposed to the US-led coalition involving more than sixty countries and its lack of seriousness in fighting terrorism and the very little it has achieved since its establishment one and a half years ago," Assad added.

Palmyra, known as the "bride of the desert", used to attract tens of thousands of tourists a year before the conflict started in 2011.

The army was now trying to dismantle booby trap bombs in some of the city's neighbourhoods, sources told Al Jazeera.

"We know from previous battles that ISIL may try to plant IEDs (improvised explosive devices) and booby traps in some homes before they leave," Al Jazeera's Omar al-Saleh, reporting from the Syrian-Turkish border, said.

In May, ISIL took Palmyra in a matter of days and later demolished some of its best-known monuments. The fate of the remaining artifacts is unknown.

The group also destroyed the infamous Tadmur prison, where thousands of government opponents were reported to have been tortured.

Capturing the city will be seen as a major strategic victory for President Bashar al-Assad's government. It opens up for a possible advance much of the eastern desert, stretching to the Iraqi border to the south and ISIL's heartlands of Deir al-Zor and Raqqa to the east.

With Russian support, Assad's troops have made steady gains in recent months against ISIL, also known as ISIS, and other groups it is fighting.

Earlier this month, Russian President Vladimir Putin ordered the withdrawal of some fighter jets from Syria, but said that strikes against ISIL and another group, the Nusra Front, would continue.

Those groups have been excluded from a Russian and US-brokered ceasefire that began on February 27 and has largely held.