The US has pulled out its ambassador to Syria, Robert Ford, because of concerns for his safety, officials say.
Mr Ford angered Syrian authorities by showing solidarity with activists involved in an uprising against President Bashar al-Assad's rule.
A US spokesman said Mr Ford had left Damascus at the weekend after "credible threats against his personal safety".
In response, Syria said it had recalled its ambassador to Washington for consultations.
Ambassador Imad Moustapha left Washington on Monday, an embassy spokeswoman told the Associated Press.
In Syria last month, Mr Ford and colleagues were pelted with eggs and tomatoes when visiting an opposition figure.
He was then briefly trapped in his office by pro-Assad demonstrators.
An official at the US embassy in Damascus told the Associated Press Mr Ford had not been formally recalled.
Mark Toner, a spokesman for the US state department, said Mr Ford's return to Syria would depend on an "assessment of Syrian regime-led incitement and the security situation on the ground".
"We hope that the Syrian regime will end its incitement campaign against Ambassador Ford," he said.
Mr Ford, an Arabic-speaker who has served in several Arab countries, has expressed solidarity with protesters as well as denouncing Syria's crackdown on its opponents.
He has warned of the risk of sectarian conflict developing in Syria if the authorities intensify their suppression of protests.
'US software used'
In July the ambassador visited the restive city of Hama in July along with his French counterpart, where he met demonstrators.
Last month he was among a group of mainly Western diplomats who paid condolences to the family of a human rights activist allegedly killed under torture.
Protests against President Assad's rule began in March in southern Syria and gradually spread across the country.
According to the UN, more than 3,000 people - mostly unarmed demonstrators - have been killed since then.
There continue to be daily reports of activists and protesters dying.
Mr Ford arrived in Damascus in January as the first US ambassador to Syria for more than five years.
The US has passed several rounds of sanctions against President Assad's government, as well as pushing for a UN resolution.
However, US newspapers reported on Sunday that the Syrian government had been using software developed by a California-based company to censor the internet and conduct surveillance.
US officials said they were looking into the reports.
The company, Blue Coat Systems, told the Washington Post that it had not sold software to Syria's government, but left open the possibility that authorities there could have obtained it through a third party.