* Ancient Palmyra is home to 2,000-year-old monuments
* Syrian antiquities chief says world must save site
* Attack is part of Islamic State westward advance 

Reuters/Beirut

Islamic State militants stormed the Syrian city of Palmyra on Wednesday, seizing it from government forces in fierce fighting as civilians were evacuated and Syria's antiquities chief called on the world to save its ancient monuments.

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.

It is home to modern military installations, and sits on a desert highway linking the capital Damascus with Syria's eastern provinces, mostly under rebel control.

"Praise God, (Palmyra) has been liberated," said an Islamic State fighter speaking by Internet from the 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.

Syrian state television said pro-government National Defence Forces (NDF) had evacuated civilians after large groups of Islamic State fighters entered the city from the north.

"The news at the moment is very bad. There are small groups that managed to enter the city from certain points," Syria's antiquities chief Maamoun Abdulkarim told Reuters earlier on Wednesday.

Abdulkarim said 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."

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

UNIQUE CULTURAL HERITAGE

UNESCO called for an immediate halt to the fighting and called for international efforts to protect the population "and safeguard the unique cultural heritage".

The Syrian military said in a statement Islamic State had entered a northern district of Palmyra and NDF fighters were trying to stop them "infiltrating" other areas. It said a Syrian air force strike in the area had killed and wounded "dozens of terrorists" and destroyed five vehicles.

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

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 Syria's 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.