Islamic State seized full control of the historic city of 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 throw doubt on US 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 it was in full charge of Palmyra, including its military bases, marking the first time it had taken a city directly from President Bashar al-Assad's army and allied forces.

The Britain-based Syrian Observatory for Human Rights said Islamic State now controls more than half of Syrian territory following four years of civil war.

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 is very strategically situated and can now be used as a launching pad for further territorial pushes towards Homs and Damascus," said Matthew Henman, head of IHS Jane's Terrorism and Insurgency Centre, in a note.

Palmyra's fall came just five days after the Islamist group seized Ramadi, capital of Iraq's largest province, Anbar. Fighters loyal to the group have also consolidated their grip on Sirte in Libya, hometown of former leader Muammar Gaddafi, extending their reach in the region.

COUNTER-OFFENSIVE

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 US-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 US 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. "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.

Sabah Nouri, spokesman for Iraq's counterterrorism forces, said forces were "flowing to Habbaniya with new lethal arms that will help reverse the course of action against Daesh".

He said the reinforcements would allow the army to wage several simultaneous attacks, and that regular troops would hold any recaptured territory to allow the counter-terrorism units to focus on maintaining an offensive.

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.

In Syria's northeast, Kurdish forces backed by US-led air strikes have been pressing an attack on Islamic State in Hasaka province, which links territories held by the group in Syria with neighbouring Iraq. Scores of its members have been killed this week according to the Observatory and a Kurdish official. – Reuters