DAMASCUS - US President Barack Obama admitted that he still does not have a strategy to fight Islamic State jihadists in Syria, as the militants boasted they had executed scores of Syrian troops -the latest in a string of atrocities that have shocked the world.
Dampening hopes of imminent air strikes in Syria, Obama said he was still developing a comprehensive plan to defeat IS, which has also overrun large swathes of Iraq.
The civil war in Syria has killed some 191,000 people since it erupted in March 2011 with President Bashar al-Assad's bloody effort to put down an uprising.
But it has taken on another dimension as IS jihadists exploited the power vacuum to move in, unleashing a series of atrocities including the brutal execution of US journalist James Foley.
The chaotic situation on the ground was underlined by the seizing of 43 UN peacekeepers on the Golan Heights by rival Islamist rebels, led by Al-Qaeda affiliate Al-Nusra Front.
They were part of a mission that has monitored an armistice between Syrian and Israeli troops on the strategic plateau for decades.
"We don't have a strategy yet," Obama said ahead of a meeting with security chiefs.
But he said he was dispatching Secretary of State John Kerry to the Middle East to build support in the region against the IS jihadists.
The jihadists posted grisly video footage on the Internet of scores of bodies heaped in the desert they boasted were those of Syrian soldiers they captured and killed following the seizure of Tabqa air base.
They have repeatedly posted gruesome videos, which have appalled international opinion but served as a propaganda tool to recruit volunteers.
Britain-based monitoring group the Syrian Observatory for Human Rights said IS had executed at least 160 soldiers, among some 500 who had made a desperate bid to escape to government-held territory after their defeat last Sunday.
'No need to choose between IS and Assad'
The footage posted by IS showed a close-up of some 20 bodies, but then panned out to show scores more.
Other shots showed men barefoot and dressed only in their underwear walking in line with their hands on their head in surrender, escorted by jihadist gunmen.
One held up the black flag of the jihadists. Others chanted: "Islamic State forever."
Tabqa was the last position in Raqa province to fall to the jihadists, who now control a vast swathe of northeastern Syria and Iraq.
A UN-mandated probe has charged that public executions, amputations, lashings and mock crucifixions have become a regular fixture in jihadist-controlled areas of Syria.
The Syrian government launched air strikes of its own on Thursday, killing six IS leaders, the Observatory said, but Washington has baulked at cooperating with Damascus against the group.
On Thursday, Obama reiterated the stance, saying: "I don't think there's a situation where we have to choose between Assad or the kinds of people who carry on the incredible violence that we've been seeing there."
UN peacekeepers held
Rival jihadists of Al-Nusra Front, backed by other rebels, detained 43 Fijian peacekeepers on the Golan on Thursday, a day after their capture of the sole crossing over the UN-patrolled armistice line to the Israeli-occupied sector of the plateau.
"Forty-three peacekeepers from the United Nations Disengagement Observer Force were detained early this morning by an armed group in the vicinity of Quneitra," a UN statement said.
The 43 peacekeepers from Fiji were forced to surrender their weapons and taken hostage, but 81 Filipino blue helmets "held their ground" and refused to disarm, the Philippine defence department said.
It said the Filipino peacekeepers were being surrounded by the gunmen in a stand-off.
The UN Security Council and the US separately condemned the action and demanded the "unconditional and immediate release" of the peacekeepers, a statement said.
Peacekeepers were detained twice last year before being released safely.
The Philippines has said it will repatriate its 331-strong contingent, for security reasons, mirroring previous moves by Australia, Croatia and Japan.
Washington has been weighing both aid drops and air strikes in Iraq to help residents of a Shiite Turkmen town besieged by the jihadists since early June, US officials said on Wednesday.
"It could be a humanitarian operation. It could be a military operation. It could be both," said a US defence official on condition of anonymity.
UN Iraq envoy Nickolay Mladenov has called for an urgent effort to help Amerli, saying residents face a "possible massacre" if the town is overrun.
The Washington Post reported Thursday that at least four Western hostages held by the Islamic State in Syria, including the murdered Foley, were waterboarded in the early part of their captivity.
Waterboarding, which was used by the CIA during interrogations of suspected terrorists after the September 11, 2001 attacks, is a widely condemned form of torture that simulates drowning.
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Obama says ''no strategy yet'' as IS executes Syrian troops