Islamic State says it has full control of Syria's Palmyra

The twin successes not only pile pressure on Damascus and Baghdad but throw doubt on a US strategy of relying almost exclusively on air strikes to support the fight against Islamic State.

US and coalition forces had conducted 18 air strikes against Islamic State targets in Syria and Iraq since May 20, the US military said.

The British-based Syrian Observatory for Human Rights said the al Qaeda offshoot now controlled more than half of all Syrian territory after more than four years of conflict that grew out of an uprising against President Bashar al-Assad.

The monitoring group added that Islamic State had seized the last border crossing between Syria and Iraq controlled by the Damascus government. The crossing is in Syria's Homs province, where Palmyra is located.

Fighters loyal to the Sunni Muslim group have also consolidated their grip on the Libyan city of Sirte, home town of former leader Muammar Gaddafi.

The White House said the seizure of Palmyra was a setback for US-led coalition forces in their fight against Islamic State. But spokesman Josh Earnest said President Barack Obama disagreed with Republicans demanding he send ground troops to fight the Islamist militants.

The Obama administration has publicly expressed confidence in Iraqi Prime Minister Haider al-Abadi, but some US officials are questioning privately whether he is too weak to bridge Iraq's sectarian divide.

Islamic State said in a statement posted by followers on Twitter that 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 UN human rights office in Geneva said a third of Palmyra's 200,000 residents may have fled the fighting in the past few days.