Islamic State fighters tightened their grip on the historic Syrian city of Palmyra on Thursday and overran Iraqi government defenses east of Ramadi, the provincial capital that they seized five days earlier.

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 Wednesday, 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 theDamascus 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 U.S. 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.

FEARS FOR CIVILIANS

UN human rights spokeswoman Ravina Shamdasani also said there were reports of government forces preventing civilians leaving, although state media said pro-government National Defense Forces had evacuated civilians before withdrawing.

"ISIL (Islamic State) has reportedly been carrying out door-to-door searches in the city, looking for people affiliated with the government," Shamdasani said. "At least 14 civilians are reported to have been executed by ISIL in Palmyra this week."

The ultra-hardline group has destroyed antiquities 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 UN cultural agency, UNESCO, describes the site as a historical crossroads between the Roman Empire, India, Chinaand 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's director general, Irina Bokova, told Reuters Television.

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

Al-Azhar, the center of Islamic learning in Egypt, urged the world to protect Palmyra, saying the destruction of cultural heritage was forbidden by Islam.

Rami Abdulrahman, founder of the Syrian Observatory for Human Rights, said Islamic State fighters had entered the ancient sites by early on Thursday but there were no immediate reports of destruction.

WESTWARD ADVANCE

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

Taking Palmyra gives Islamic State access to modern army installations and control of a desert highway linking government-held Damascus and Homs with Syria's mainly rebel-held east.

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 on the coast.

Just five days before Palmyra fell, Islamic State seized Ramadi, capital of Iraq's largest province, Anbar, where the Sunni Muslim Islamic State has tapped into resentment among local Sunnis who say they have been marginalized by Shi'ite-led governments in Baghdad.

Obama said the fall of Ramadi was a "tactical setback" but, in an interview released on Thursday, added that he did not think the fight against Islamic State was being lost.

'MILITARY, DIPLOMATIC AND ECONOMIC HELP'

"There's no doubt that, in the Sunni areas, we're going to have to ramp up not just training, but also commitment, and we

better get Sunni tribes more activated than they currently have been," Obama said in the interview, conducted on Tuesday with The Atlantic magazine.

"I think Prime Minister Abadi is sincere and committed to an inclusive Iraqi state, and I will continue to order our military to provide the Iraqi security forces all assistance that they need in order to secure their country, and I'll provide diplomatic and economic assistance that's necessary for them to stabilize," Obama said.

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

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

One US official, speaking on condition of anonymity, said there had been long-standing concerns in Washington about Abadi's ability to navigate Iraq's sectarian politics and that recent events had raised misgivings about him.

But Washington sees no viable alternative for Abadi, current and former US officials said.