State Department spokesman Mark Toner said Monday that Ambassador Robert Ford returned to Washington this weekend after "credible threats against his personal safety".
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Ford has not been formally recalled, Toner said, adding that the envoy would return if the security situation allowed it.
Toner referred to "Syrian regime-led incitement", suggesting that President Bashar Assad's government is suspected to be behind the threats.
One unnamed diplomat told the news agency:
"Articles, more inciting against Ford than usual, have appeared in state media recently."
His residence and embassy have been attacked, and last month Ford was pelted with eggs and tomatoes as he visited a member of the Syrian opposition.
At least 3,000 people are said to have died in protests in Syria since March, according to the United Nations.
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