Islamic State group jihadists seized full control of the ancient Syrian city of Palmyra on Thursday, putting the world heritage site and its priceless artefacts at risk of destruction.
The ransacking of Palmyra, a 2,000-year-old former stopping point for caravans on the Silk Road, would be "an enormous loss to humanity," UNESCO head Irina Bokova warned.
The Britain-based Syrian Observatory for Human Rights said IS now controlled half of all territory in the war-torn country.
The capture of Palmyra is the latest blow to efforts to hold back the advancing jihadists, following the fall of Iraq's Ramadi.
"IS fighters are in all parts of Tadmur, including near the archeological site," Observatory head Rami Abdel Rahman told AFP, using the Arabic name for the city.
Syrian state media said loyalist troops withdrew after "a large number of IS terrorists entered the city".
In an online statement, IS proclaimed its capture of the entire city, which is strategically located at the crossroads of key highways leading west to Damascus and Homs, and east to Iraq.
The Observatory, which relies on a network of sources on the ground, said some residents had headed to the city of Homs but others stayed home, while state media said most of Palmyra's civilians had been evacuated.
IS also seized Palmyra's prison, notorious for the killings of hundreds of regime prisoners in the 1980s and seen as a symbol of oppression during the reign of President Bashar al-Assad's father Hafez al-Assad.
The jihadists, notorious for demolishing archaeological treasures since declaring a "caliphate" last year straddling Iraq and Syria, fought their way into Palmyra on foot after breaking through in the city's north.
- 'Birthplace of civilisation' -
Known in Syria as "the pearl of the desert," Palmyra is home to colonnaded alleys, elaborately decorated tombs, and ancient Greco-Roman ruins.
"At the end of the day, it's the birthplace of human civilisation. It belongs to the whole of humanity and I think everyone today should be worried about what is happening," Bokova said.
The jihadists sparked international outrage this year when they blew up the ancient Assyrian city of Nimrud and smashed artefacts in the museum of Mosul, both in Iraq.
Before Syria's conflict began in March 2011, more than 150,000 tourists visited Palmyra every year
- 'What went wrong?' -
"Palmyra was the number one destination in Syria for all international tour operators," said Maha Qandalaft at Syrian travel agency Adonis.
The pro-government newspaper Al-Watan said Palmyra's capture "shames the international community, which crossed its arms as IS infiltrated the most celebrated historical city in the world".
Fierce clashes in and around Palmyra have cost the lives of at least 462 people, including 71 civilians, 241 regime forces, and 150 jihadists, the Observatory said.
By taking Palmyra, which sits 210 kilometres (130 miles) northeast of Damascus in desert that extends to the Iraqi frontier, IS controls more than 95,000 square kilometres of Syria, according to the monitor.
The jihadist group dominates the provinces of Deir Ezzor and Raqa and has a strong presence in Hasakeh, Aleppo, Homs, and Hama.
It has also seized most of Syria's oil and gas fields, using the income to fund the expansion of its self-styled "caliphate".
The fall of Palmyra came days after the militants took the Iraqi city of Ramadi, their most significant victory since mid-2014 when they conquered swathes of land, sparking a US-led air campaign to support Baghdad.
A US State Department official said the loss of Ramadi would force Washington to take an "extremely hard look" at its strategy against IS.
"You'd have to be delusional not to take something like this and say: 'What went wrong, how do you fix it and how do we correct course to go from here?'," the official told reporters.
The official said Washington would step up its aid to Iraq, including sending 1,000 anti-tank missile systems to help stop suicide car bombs and accelerating its training and equipping of tribal forces to fight IS.
On Thursday, Iraqi forces backed by powerful Shiite militias prepared to launch an offensive to retake Ramadi.