Islamic State militants have seized control of one third of the ancient Syrian city of Palmyra as hundreds of statues and artefacts are moved to safer locations outside the world heritage site.

The militants took control of Palmyra's northern sector after fierce clashes with government troops on the edge of the city.

It is the second time IS has overrun northern Palmyra in recent days. On Saturday militants seized the same neighbourhoods but were forced out within 24 hours.

Rami Abdel Rahman, the head of the Syrian Observatory for Human Rights, said the latest IS offensive had seen the militants take control of "a third of Palmyra".

He added that IS seized a state security building before spreading out across northern districts as regime forces fled.

"People are very afraid of what will happen, because IS has the capability to get to the heart of Palmyra," said Khaled al Homsi, an activist in the city.

Palmyra's UNESCO world heritage site ruins, including ancient temples and colonnaded streets, are located in the city's southwest.

Hundreds of statues and ancient artefacts have been removed from Palmyra's museum amid fears they will be destroyed by IS fighters.

Syrian antiquities director Mamoun Abdulkarim said many items, including large stone tombs, cannot be moved.

"The situation is very bad. If only five members of IS go into the ancient buildings, they'll destroy everything," he said.

The militant group has already destroyed antiquities at the ancient Assyrian city of Nimrud and the Roman-era city of Hatra.

IS began its assault on Palmyra on 13 May by seizing a nearby town and two gas fields, and killing more than 350 people.

The city is located at a strategic crossroad for key highways leading to Damascus and Homs, and east into Iraq.