The twin successes pile pressure not just on Damascus and Baghdad, but also throw doubt on U.S. strategy to rely almost exclusively on air strikes to defeat Islamic State.

Extending its reach in the region, fighters loyal to the Sunni Muslim group have also consolidated their grip on the Libyan city of Sirte, hometown of former leader Muammar Gaddafi.

Islamic State said in a statement posted by followers on Twitter on Thursday it was in full charge of Palmyra, including its military bases, marking the first time it had taken a city directly from the Syrian military and allied forces.

The Britain-based Syrian Observatory for Human Rights said

the al Qaeda offshoot now controls more than half of Syrian territory following more than four years of conflict which grew out of an uprising against President Bashar al-Assad.

The ultra hardline group has destroyed antiquities and monuments in Iraq and there are fears it might now devastate Palmyra, home to renowned Roman-era ruins including well-preserved temples, colonnades and a theater.

The U.N. cultural agency UNESCO describes the site as a historical crossroads between the Roman Empire, India, China and ancient Persia and a testament to the world's diverse heritage.

"We may have different beliefs... different views, but we have to protect such incredible vestiges of human history," UNESCO Director General Irina Bokova told Reuters Television.

The Observatory's founder Rami Abdulrahman said Islamic State fighters had entered the historical sites by early on Thursday but there were no immediate reports of destruction.

"This is the fall of a civilization," Syria's antiquities chief Maamoun Abdulkarim told Reuters. "Human, civilized society has lost the battle against barbarism."

"WAR CRIMES

Al-Azhar, the center of Islamic learning in Egypt, called on the world to protect Palmyra, saying the destruction or looting of cultural heritage was religiously forbidden.

Clashes in the Palmyra area since Wednesday killed at least 100 pro-government fighters, said Abdulrahman, who bases his information on a network of sources on the ground.

Syrian state media said pro-government National Defense Forces had evacuated civilians before withdrawing.

The assault on the city is part of a westward advance by Islamic State that is adding to pressures on Syria's overstretched army and militia, which have also recently lost ground in the northwest and south.

Capturing the city marks a strategic military gain for Islamic State, because it is home to modern army installations and situated on a desert highway linking government-held Damascus and Homs with Syria's mainly rebel-held east.

The European Union's foreign policy chief expressed fears that thousands of people in Palmyra were at risk as well as the cultural sites.

"Mass killings and deliberate destruction of archaeological and cultural heritage in Syria and Iraq amount to a war crime," Federica Mogherini said in a statement.

Although Islamic State has seized large chunks of Syria, the areas it holds are mostly sparsely inhabited. Syria's main cities, including the capital Damascus, are located on its western flank along the border with Lebanon and the coastline.

In the northeast, Kurdish forces backed by U.S.-led air strikes have been pressing an attack on Islamic State in Hasaka province, which links land held by the group in Iraq. Scores of its members have been killed this week and state television also reported progress by the army in the area on Thursday.

IRAQ OFFENSIVE

Palmyra's fall came just five days after the Islamist group seized Ramadi, capital of Iraq's largest province, Anbar.

Iraqi forces said on Thursday that they had thwarted a third attempt by Islamic State militants to break through their defensive lines east of Ramadi overnight.

Police and pro-government Sunni fighters exchanged mortar and sniper fire with insurgents across the new frontline in Husaiba al-Sharqiya, about halfway between Ramadi and a base where a counter-offensive to retake the city is being prepared.

The loss of Ramadi handed the Iraqi government its most significant setback in a year and exposed the limitations of both the army and U.S.-led air strikes designed to "degrade and destroy" Islamic State.

The United States plans to deliver 1,000 anti-tank weapons to Iraq in June to combat suicide bombings like those that helped the Islamist group seize Ramadi, a senior U.S. State Department official said on Thursday.

Iraq's government has ordered Shi'ite militia, some of which have close ties to Iran, to join the battle to retake Ramadi, raising fears of renewed sectarian strife in the country.

Washington wants the counter-offensive to include both Sunni and Shi'ite forces under the direct government command.

The militants in Ramadi are seeking to consolidate their gains in Anbar province by pushing east to the Habbaniya base where security forces and Shi'ite paramilitaries are massing.

"Daesh is desperately trying to breach our defenses but this is impossible now," Police major Khalid al-Fahdawi said, referring to Islamic State. "They tried overnight to breach our defenses but they failed. Army helicopters were waiting for them."

Habbaniya is one of only a few remaining pockets of government-held territory in Anbar, and lies between Ramadi and the town of Falluja, which has been controlled by Islamic State for more than a year.

(Additional reporting by Mariam Karouny in Beirut, Kinda Makieh in Damascus and Isabel Coles in Erbil; Writing by Crispian Balmer; Editing by Dominic Evans)

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