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.