The Obama administration told the Wall Street Journal. it had "credible" evidence that the regime of Syrian President Bashar al-Assad is using the reports from its embassies abroad to target relatives of those living overseas, particularly Syrian-Americans who have joined peaceful US protests.

On Wednesday' the Journal, citing interviews with six Syrian-Americans, said embassy staffers were tracking and photographing protesters, and that Syrian diplomats including the ambassador to Washington have gone to Arab diaspora communities to brand dissidents as "traitors."

"They want to intimidate us wherever we are," Philadelphia-based Syrian-American scientist Hazem Hallak told the daily.

Hallak said his brother Sakher was tortured and killed in May by Syrian intelligence after he returned from a conference in the United States. Hallak said agents in the Syrian city of Allepo sought to obtain a list of activists and US officials that Sakher had allegedly met during his US stay, and that Syrian agents tracked his brother in the United States.

He said his brother was not involved in anti-regime activities.

The daily, citing three people interviewed by the FBI in recent weeks, also said the Federal Bureau of Investigation was probing allegations that Syrian Ambassador Imad Mustapha and embassy staff have threatened Syrian-Americans.

The US State Department publicly rebuked Mustapha last month after reports that embassy staff were "conducting video and photographic surveillance of people participating in peaceful demonstrations in the United States."

On Tuesday in an interview with the Journal, Mustapha dismissed the allegations by Syrian-Americans and US officials as "slander and sheer lies," and that "the Embassy of Syria challenges the State Department to provide a single shred of evidence that the embassy has harassed or conducted surveillance on anyone."

The paper cited several incidents of intimidation by Syrian officials against dissidents in the United States, as well as in Europe and Latin America.

Rights groups say the ongoing crackdown in Syria has killed 1,827 civilians since mid-March, while 416 security forces have also died.