* Ancient Palmyra is home to 2,000-year-old monuments

* Syrian antiquities chief says world must save site

* Attack is part of Islamic State's westward advance (Adds withdrawal of pro-government fighters)

By Sylvia Westall and Tom Perry

BEIRUT, May 20 (Reuters) - Islamic State insurgents stormed the historic Syrian city of Palmyra on Wednesday, fighting off pro-government forces who withdrew after evacuating most of the civilian population, state television said.

The capture of Palmyra is the first time the al Qaeda offshoot has taken control of a city directly from the Syrian army and allied forces, which have already lost ground in the northwest and south to other insurgent groups in recent weeks.

The central city, also known as Tadmur, is built alongside the remains of a oasis civilisation whose colonnaded streets, temple and theatre have stood for 2,000 years.

Islamic State has destroyed antiquities and ancient monuments in neighbouring Iraq and is being targeted by U.S.-led air strikes in both countries.

Syria's antiquities chief called on the world to save its ancient monuments and state television said Islamic State fighters were trying to enter the city's historical sites.

Palmyra is also a strategic military gain, home to modern army installations and situated on a desert highway linking the capital Damascus with Syria's eastern provinces, mostly under insurgent control.

"Praise God, it has been liberated," said an Islamic State fighter speaking via the Internet from the Palmyra area. He said Islamic State was in control of a hospital in the city which Syrian forces had used as a base before withdrawing.

The Syrian Observatory for Human Rights monitoring group said Islamic State had seized almost all of the city. It said it was unclear what had happened to forces stationed at an army outpost on its outskirts or the fate of a major military prison.

Syria's antiquities chief Maamoun Abdulkarim told Reuters earlier on Wednesday that hundreds of statues had been moved to safe locations but called on the Syrian army, opposition and international community to save the site.

"The fear is for the museum and the large monuments that cannot be moved," he said, "This is the entire world's battle."

WESTWARD MARCH

The attack is part of a westward advance by Islamic State that is adding to pressures on the overstretched military and allied militia. The group holds tracts of land in the north and east and is now edging towards the more heavily populated areas along its western flank.

In the east, U.S. special forces carried out a ground assault on Saturday against Islamic State and killed a militant believed to be in charge of the group's financial operations.

UNESCO called for called for international efforts to protect Palmyra's population "and safeguard the unique cultural heritage".

Palmyra's ancient monuments, which lie on the south-western fringe of the modern city, were put on UNESCO's World Heritage in danger list in 2013. The ruins were part of a desert oasis that was one of the most significant cultural centres of the ancient world.

Islamic State supporters posted pictures on social media showing what they said were gunmen in the streets of Palmyra, which is the location of one of Syria's biggest weapons depots as well as army bases, an airport and a major prison.

In the northeast, Kurdish forces backed by U.S.-led air strikes pressed an attack on Islamic State that has killed at least 170 members of the group this week, a Kurdish official and the Observatory said. U.S-led forces have concentrated their air strikes on Syria's north and east, areas out of government control. (Additional reporting by Kinda Makieh in Damascus and Mariam Karouny in Beirut; Editing by Louise Ireland)

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