* Islamic State enters famed site of ancient Palmyra
* Radical group controls half of Syria - monitors
* Islamist movement consolidates in Iraq after gains there
BAGHDAD, May 21 (Reuters) - Islamic State seized full control of both ancient and modern Palmyra in central Syria on Thursday, just days after it captured a provincial capital in neighbouring Iraq, suggesting momentum is building for the ultra-hardline group.
The twin successes pile pressure not just on Damascus and Baghdad, but also throws doubt on U.S. strategy to rely almost exclusively on air strikes to defeat the Sunni Muslim movement, which is an offshoot of al Qaeda.
Islamic State said in a statement posted by followers on Twitter that it was in full charge of Palmyra, including its military installations, 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 Islamic State now controls more than half of Syrian territory following more than four years of civil war against the autocratic rule of President Bashar al-Assad.
The radical group has destroyed antiquities and monuments in Iraq and there are fears it might now devastate Palmyra, an ancient World Heritage site and home to renowned Roman-era ruins including well-preserved temples, colonnades and a theatre.
"This is the fall of a civilisation," Syria's antiquities chief Maamoun Abdulkarim told Reuters by telephone on Thursday. "Human, civilised society has lost the battle against barbarism. I have lost all hope."
Clashes in the area since Wednesday killed at least 100 pro-government fighters, said Rami Abdulrahman, head of the Syrian Observatory for Human Rights, which bases its information on a network of sources on the ground.
Islamic State said retreating pro-government forces had left behind many dead, but gave no precise figures.
The assault on the city is part of a westward advance by Islamic State that is adding to pressures on Assad's overstretched army and pro-government militia, which have also recently lost ground in the northwest and south.
Palmyra's fall came just five days after the Islamist group seized Ramadi, capital of Iraq's largest province, Anbar. Fighters loyal to the movement have also consolidated their grip on Sirte in Libya, hometown of late dictator Muammar Gaddafi, extending their reach in the region.
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 central Iraq government in Baghdad its most significant setback in a year and exposed the limitations of both the Iraqi army and a campaign of 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 grab 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 has said it will support the Ramadi counter-offensive, but says it should include both Sunni and Shi'ite forces under the direct command of central government.
The militants in Ramadi are seeking to consolidate their gains in the surrounding province of Anbar by pushing east towards the Habbaniya base where Iraqi security forces and Shi'ite paramilitaries are massing.
"Daesh is desperately trying to breach our defences but this is impossible now," Police major Khalid al-Fahdawi said, referring to Islamic State. "We have absorbed the shock and more reinforcements have reached the frontline. They tried overnight to breach our defences 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.
Local officials say the militants want to join up the two towns and overrun the other remaining government holdouts, strung out along the Euphrates river valley and the border with Jordan and Saudi Arabia.
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 and have been the priority for Assad's government.
But the capture of Palmyra marks a strategic military gain for Islamic State, because it is home to modern army installations and situated on a desert highway connecting the government-held cities of Damascus and Homs with Syria's mostly rebel-held eastern provinces. (Reporting by Sylvia Westall and Mariam Karouny in Beirut, Kinda Makieh in Damascus, and Isabel Coles; Writing by Crispian Balmer; Editing by Dominic Evans)