DAMASCUS (AFP) - The Islamic State in Iraq and Syria (ISIS) group consolidated its control of the Iraq-Syria border Friday after capturing an Iraqi provincial capital and a famed Syrian heritage site in an offensive that has forced a review of US strategy.
The militants, who now control roughly half of Syria, reinforced their self-declared transfrontier "caliphate" by seizing Syria's Al-Tanaf crossing on the Damascus-Baghdad highway late Thursday.
It was the last regime-held border crossing with Iraq. Except for a short section of frontier in the north under Kurdish control, all the rest are now held by ISIS.
The extremist surge, which has also seen it take Anbar capital Ramadi and the ancient Syrian city of Palmyra in the past week, comes despite eight months of US-led air strikes.
It has sparked an exodus of tens of thousands of civilians in both countries and raised fears IS will repeat at Palmyra the destruction it has already wreaked at ancient sites in Iraq's Nimrud and Mosul.
The United Nations said Friday at least 55,000 people had fled Ramadi alone since mid-May.
President Barack Obama has played down the ISIS advance as a tactical "setback" and denied the US-led coalition was "losing" to ISIS.
But French President Francois Hollande said the world must act to stop the extremists.
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."
In Palmyra, at a strategic crossroads between Damascus and the Iraqi border to the east, IS executed at least 17 suspected loyalists of the Damascus government Thursday, the Syrian Observatory for Human Rights said.
ISIS proclaimed Palmyra's capture online and posted video and footage of its fighters in the city's air base, but not of the Unesco world heritage site's colonnaded streets, elaborately decorated tombs and temples.
Syria's antiquities director Mamoun Abdulkarim urged the world to "mobilise" to save the treasures.
Also on Thursday, a Syrian priest and his colleague were kidnapped from a monastery between Homs and Palmyra, the French NGO L'Oeuvre d'Orient said.
Father Jacques Mourad was preparing aid for an influx of refugees from Palmyra and was known to help both Christians and Muslims.
ISIS now controls "more than 95,000 sq km in Syria, which is 50 per cent 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 militant advance "reinforces ISIS' position as the single opposition group that controls the most territory in Syria".
According to the Observatory, ISIS gains mean a mere 22 per cent of Syria's territory is still in regime hands.
Syria's opposition government in exile said the regime controlled "less than a quarter" of the country.
ISIS' extremist rival, Al-Qaeda affiliate Al-Nusra Front, has also been on the offensive as part of a rebel alliance that has stormed through nearly all of the northwestern province of Idlib.
On Friday, the alliance overran a hospital in Jisr al-Shughur where at least 150 regime forces and dozens of civilians were trapped for nearly a month, the Observatory said.
Dozens managed to escape but despite a pledge from President Bashar al-Assad to rescue them, others were killed, it said.
The Observatory also reported eight women and three young girls killed when regime forces dropped two barrel bombs on their village in the northern province of Aleppo.
Palmyra's takeover came days after ISIS seized the Iraqi city of Ramadi, their most significant victory since last summer's lightning advance across swathes of northern Iraq.
Officials said ISIS also seized Iraqi positions east of Ramadi on Thursday, but added that government forces were preparing a counteroffensive to retake the city "in the coming days".
Iraqi forces cleared a ground route to the country's largest oil refinery at Baiji, under siege by ISIS for months, the US military said on Friday.
Obama blamed the fall of Ramadi on a lack of training and reinforcements for its garrison, saying Iraqi security forces in "Sunni parts of the country" needed speedier support.
"I don't think we're losing," he told news magazine The Atlantic.
One of Iraq's leading Sunni Arab politicians, Deputy Prime Minister Saleh Mutlaq, called for a change of US strategy.
Recruiting Sunni tribes is "important but not enough," he said, adding that in any case it was "too late".