Syria is taking its war against President Bashar al-Assad's political opponents global, using diplomats in Washington, London and elsewhere to track and intimidate expatriates who speak out against the Damascus regime, according to Syrian dissidents and U.S. officials.

Syrian embassy staffers are tracking and photographing antiregime protesters and sending reports back home, Syrian activists and U.S. officials say. Syrian diplomats, including the ambassador to the U.S., have fanned out to Arab diaspora communities to brand dissidents "traitors" and warn them against conspiring with "Zionists."

A half-dozen Syrian-Americans interviewed by The Wall Street Journal in recent weeks say that as a result of their activities in the U.S., family members have been interrogated, threatened or arrested in Syria. The Obama administration says it has "credible" evidence that the Assad regime is targeting relatives of Syrian-Americans who have participated in peaceful U.S. protests.

In an interview Tuesday, Imad Moustapha, the Syrian ambassador, dismissed the allegations by Syrian dissidents and U.S. officials as "slander and sheer lies."

One Syrian-American scientist in Philadelphia, Hazem Hallak, said his physician brother, Sakher, was tortured and killed in May by Syria's intelligence agencies, the mukhabarat, after he returned from a medical conference in the U.S. Syrian agents in Aleppo were obsessed with obtaining a list of Syrian activists and U.S. officials the brother had allegedly met during his stay, Hazem Hallak said.

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"They want to intimidate us wherever we are," said Mr. Hallak, who said he believes Syrian agents or regime sympathizers tracked his brother inside the U.S. Mr. Hallak said his brother wasn't involved in anti-Assad activities.

The State Department recently publicly rebuked the Syrian ambassador, Mr. Moustapha, for allegedly intimidating activists and confined him to a 25-mile radius around Washington.

"We received reports that Syrian mission personnel under Ambassador Moustapha's authority have been conducting video and photographic surveillance of people participating in peaceful demonstrations in the United States," the State Department said. "The United States Government takes very seriously reports of any foreign government actions attempting to intimidate individuals in the United States who are exercising their lawful right to freedom of speech as protected by the U.S. Constitution."

The Federal Bureau of Investigation, meanwhile, is investigating allegations that Mr. Moustapha and his staff have threatened or harmed Syrian-Americans, according to three individuals interviewed by the FBI in recent weeks. An FBI spokesman said the bureau won't comment on any possible investigation into the Syrian embassy's activities.

Ambassador Moustapha is having none of it. "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," he said by telephone from Damascus, where he said he is on vacation. "We challenge any authority or organization that has extended such a ridiculous and preposterous claim to provide proof."

Asked if he was aware his travel inside the U.S. had been limited to a 25-mile radius around Washington, Mr. Moustapha said, "This is true, and we did the same to the American ambassador here" in Damascus. He called the U.S. move "reciprocity."

Click here to read more on Syria's global effort to stamp out rebellion from The Wall Street Journal.