The latest diplomatic crisis between Damascus and Washington came as 16 people were reportedly killed in new violence, including eight civilians in the central protest hub city of Homs and soldiers in clashes with army deserters. “Syria’s ambassador to the United States, Imad Mustapha, will leave Washington for Damascus to hold consultations with Syrian leaders,” Al-Ikhbariya TV said.

Earlier a US Embassy official in Damascus said that ambassador Robert Ford, an open critic of President Bashar Assad’s crackdown on political dissent, has left Syria indefinitely for security reasons.

The US State Department said Ford was pulled out due to “credible threats” against his safety. “Ambassador Robert Ford is on leave indefinitely. Washington decided to give him the leave out of concerns about his personal safety,” the embassy official said.

Ford has come in for heavy criticism by regime supporters in Damascus who have accused him of helping incite violence in the country, where according to UN estimates more than 3,000 people have been killed since mid-March.

State Department deputy spokesman Mark Toner said Ford was brought back to Washington because of “credible threats against his personal safety in Syria.”

And State Department spokeswoman Victoria Nuland said commentary about Ford in official Syrian newspapers led to the decision to pull him out.

The media are “going after him personally, spreading lies about what he is up to. And the concern is that this could lead to further violence,” she said.

Nuland said Ford would return to Damascus “after his consultations are completed” in Washington, stressing however that Syria had international obligations to ensure the ambassador’s safety.

On Monday, Syrian forces in Homs raked several neighborhoods with heavy artillery fire. The Syrian Observatory for Human Rights said eight people were killed in Homs while another civilian died when a rocket-propelled grenade fired by troops in the northwestern province of Idlib hit his car.

The Britain-based group also reported the death of two army defectors in Idlib province and said five others were killed in clashes with suspected deserters elsewhere in the province.

In the central city of Homs, 140 km north of Damascus, two people were killed when troops and loyalist militiamen fired at majority Sunni districts that have been a bastion for protests and, lately, a refuge for army defectors leading armed resistance emerging there, residents said. Syria is dominated by Assad's minority Alawite sect.

The killings bring to at least 10 the number of civilians killed in tank-backed assaults on districts in Homs in the last two days, activists said.

The official Syrian news agency said "terrorist groups" fired at a shared taxi carrying university students in Homs on Sunday night, killing a young woman. Security forces arrested several members of other groups and seized automatic weapons and automatic rifles and Molotov cocktails.

A Youtube video shot by activists purportedly showed a young protester dying from a gunshot that hit him while he was dragging a body off a street in Al-Khalidiya district. Their comrades are heard shouting "God is greater" as the two bodies lay next to each other on the asphalt.

Reuters could not confirm the authenticity of the footage. Most foreign media have been banned from Syria, making it difficult to verify events on the ground.

Syrian authorities say they are fighting "armed terrorist groups" in Homs who have killed civilians, security forces and prominent figures.

They blame the unrest across the country on such groups, which they say have killed 1,100 army and police. The United Nations says the crackdown has killed 3,000 people, including 187 children.

Anger has grown over the killings of several protesters last week in the towns of Dael and in Ibtaa. The region has seen nightly protests in solidarity with Homs.

"Troops have entered into several towns to end the strike but protesters want to expand it into wider civil disobedience," said one activist who said army reinforcements had been sent to several towns in the Deraa countryside.

In Deraa city, capital of the agricultural province, businesses across the city were closed for the fourth day. In the town of Al-Hirak to the east, the strike picked up steam in the last two days, activists said.

"This strike is intensifying every day as more businesses shut and people become more defiant than ever angered by the increasing brutality and daily roundups and arrests," said one Deraa resident who gave his name as Abu Abdullah.

With troops concentrating on urban centers, protests have expanded in rural regions, including some areas which were once bedrocks of Sunni support for Assad and are seeing defections from the military and armed resistance.