Islamic State (IS) militants have recaptured most of the historic city of Palmyra in central Syria following a surprise assault, according to a monitoring group and rebels.

Key points: IS militants "retake" Palmyra after being pushed out nine months ago

IS militants "retake" Palmyra after being pushed out nine months ago Fears for civilians inside the city

Fears for civilians inside the city IS fighters also "approaching" city's UNESCO heritage site

The militants are reported to have penetrated Syrian army defences to enter the city for the first time since being expelled by Syrian and Russian forces nine months ago.

The UK-based Syrian Observatory for Human Rights said there were fears for the lives and safety of civilians inside the city because many of them were pro-government supporters.

The activist-run Palmyra Coordination network said the militants had nearly encircled the city and entered its northern and north-western neighbourhoods.

The network, which maintains contacts inside the city, said IS fighters were approaching the city's UNESCO heritage site as well.

The network's Osama al-Khatib said Government soldiers were fleeing Palmyra, but some soldiers and militiamen remain in the city, along with 120 families who have not been able to leave.

"The army as an institution has dissolved," he said.

"There is strong fighting on all sides," he reported.

"There is no exit except through a corridor to the west."

The city had been recaptured from the militants last March, in what was hailed as a major victory for the Government and the biggest reversal for IS in Syria since Russia's intervention, which turned the tide of the conflict in President Bashar al-Assad's favour.

A statement by IS's news agency Amaq said the group had taken the strategic Jabal al Tar and Jabal Antara mountains that overlook the city in some of the heaviest fighting since the group lost the city.

A rebel contact said a large contingent of Russian troops that had been stationed in the city had been quickly pulled out.

The dramatic reversal in Palmyra comes days after IS militants in the Iraqi city of Mosul launched a major counterattack that surprised Iraqi soldiers, killing at least 20 and halting their advance.

Iraqi special forces units have entered the eastern outskirts of the largest remaining IS-held city, but their advance has been greatly slowed by both a desire to limit civilian casualties and the resilience of the IS fighters.

US Secretary of Defence Ash Carter on Saturday announced that an additional 200 American soldiers would be dispatched to Syria to accelerate the push on the self-declared IS capital of Raqqa.

During the 10 months that IS held Palmyra, from May 2015 to March 2016, the militants dynamited several of the city's famed ancient Roman monuments and executed its archaeological director.

After the city was retaken, the Russian government staged a classical music concert in the city's soaring Roman amphitheatre last May to celebrate the success.

The Syrian and Russian governments maintain they are defending the global community against Islamic terrorism in the country's devastating five-and-a-half-year war.

AP/Reuters