DAMASCUS — “Islamic State” militants tightened their grip on the historic city of Palmyra in Syria, days after capturing a provincial capital in neighboring Iraq, suggesting the growing momentum of the group which a monitor says now holds half of Syrian territory.

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 ISIS.

ISIS said in a statement posted by followers on Twitter on Thursday it was in full charge of Palmyra, which is known in Arabic as Tadmur, including its military bases, marking the first time it had taken a city directly from the Syrian military and allied forces.

Around a third of the 200,000 people living in Palmyra may have fled in the past few days during fighting between government forces and ISIS militants, the U.N. human rights office said on Thursday.

Citing what she said were credible sources, U.N. human rights spokeswoman Ravina Shamdasani in Geneva also said there were reports of government forces preventing civilians leaving until they themselves fled and ISIS, also known as IS or ISIL, took control of the city.

“ISIL has reportedly been carrying out door-to-door searches in the city, looking for people affiliated with the government. At least 14 civilians are reported to have been executed by ISIL in Palmyra this week,” Shamdasani said in emailed comments.

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 ISIS 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.

ISIS has destroyed ancient ruins and museums in northern Iraq.

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.