The Islamic State group consolidated its control of the Iraq-Syria border yesterday after capturing an Iraqi provincial capital and a famed Syrian heritage site in an offensive that has forced a review of US strategy.
The jihadists, who now control roughly half of Syria, reinforced their self-declared transfrontier "caliphate" with the capture of the Al-Tanaf to Al-Walid crossing on the Damascus-Baghdad highway late on Thursday.
It was the last border crossing with Iraq still held by the Damascus government. Except for a short section of frontier in the north under Kurdish control, all the rest are now held by ISIS.
The jihadist surge, which has also seen them capture Anbar capital Ramadi and the ancient Syrian city of Palmyra in the past week, comes despite eight months of US-led air strikes aimed at pushing them back.
It has sparked an exodus of tens of thousands of fearful civilians in both countries and raised fears that the jihadists will repeat at Palmyra the destruction they have already wreaked at ancient sites in Iraq.
President Barack Obama played down the ISIS advance as a tactical "setback" and denied the US-led coalition was "losing" to IS.
But French President Francois Hollande said the world must act to stop the extremists and save Palmyra.
UNESCO chief Irina Bokova called the 1st and 2nd Century ruins "the birthplace of human civilisation", adding: "It belongs to the whole of humanity and I think everyone today should be worried about what is happening."
As the jihadists fanned out across Palmyra on Thursday, they went door to door executing suspected loyalists of the Damascus government, the Syrian Observatory for Human Rights said.
At least 17 people were killed, the Britain-based monitoring group said.
Syrian state media said loyalist troops withdrew after "a large number of ISIS terrorists entered the city," which lies at a strategic crossroads between Damascus and the Iraqi border to the east.
ISIS proclaimed Palmyra's capture online and posted video and stills footage of its fighters in the city's air base and prison, long notorious for its detention of regime opponents.
The jihadists did not immediately post pictures of the UNESCO-listed world heritage site with its colonnaded streets, elaborately decorated tombs and temples.
The militants sparked international outrage this year when it blew up the ancient Assyrian city of Nimrud in northern Iraq.
Syria's antiquities director Mamoun Abdulkarim said he feared a similar fate awaits Palmyra, and urged the world to "mobilise" to save it.
ISIS now controls "more than 95,000 square kilometres (38,000 square miles) in Syria, which is 50 percent of the country's territory," the Observatory said.
Fabrice Balanche, a French expert on Syria, said "ISIS now dominates central Syria, a crossroads of primary importance" that could allow it to advance towards the capital and third city Homs.
Matthew Henman, head of IHS Jane's Terrorism and Insurgency Centre, said the ISIS advance boosted its claim to be the most effective of the armed groups fighting to overthrow President Bashar al-Assad.
"It reinforces ISIS's position as the single opposition group that controls the most territory in Syria," he said.
The jihadist rivals of IS, Al-Qaeda affiliate Al-Nusra Front, have also been on the offensive as part of a rebel alliance that has stormed through nearly all of the northwestern province of Idlib.
The Islamist rebels on Friday overran a hospital in the town of Jisr al-Shughur where at least 150 regime forces and dozens of civilians were trapped for nearly a month, the Observatory said.