The withdrawal of US troops from Syria depends on certain conditions, US National Security Adviser John Bolton says, in a further indication that the process is being slowed down.
On a trip to Israel and Turkey, he said he would seek Turkish assurances that Kurds in northern Syria would be safe.
The US also wants to ensure that the remnants of the Islamic State (IS) group are defeated, he added.
President Donald Trump has faced strong criticism over the planned US pullout.
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When he first announced the move in the middle of December, he said: “They’re all coming back and they’re coming back now.”
After the president made his announcement on 19 December, US officials said American forces had been given 30 days to leave Syria.
In his announcement, Mr Trump had also declared that IS had been “defeated”.
However, speaking on Sunday before leaving for his Camp David retreat, he told reporters: “We’re going to be removing our troops. I never said we were doing it that quickly.
“We’re pulling out of Syria… and we won’t be finally pulled out until Isis [IS] is gone.”
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Last month’s announcement shocked allies and US defence officials alike, with Defence Secretary Jim Mattis and senior aide Brett McGurk resigning soon after. On Saturday, Department of Defence chief of staff Kevin Sweeney became the third senior Pentagon official to announce his resignation since President Trump’s announcement.
Meanwhile, America’s Kurdish allies in north-east Syria were left feeling exposed as Turkey, which regards them as terrorists, appeared poised to move against them.
But Mr Trump seemed to row back last week when he said troops were being pulled out “slowly” and that they would be fighting remaining IS militants at the same time.
Source: BBC