The US administration has summoned the Syrian ambassador after reports that embassy staff spied on those staging protests in the US.
Eric Boswell, an assistant secretary of state, summoned Imad Mustapha on Wednesday after reports that protesters at the Syrian embassy were filmed, the state department said on Friday.
"The United States government takes very seriously reports of any foreign government actions attempting to intimidate individuals in the United States," the state department said in a statement.
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It said it is also investigating reports that the Syrian government has sought retribution against family members for the actions of their relatives protesting in the US.
Loay Safi, chairman of the Syrian American Council, which promotes civil liberties and human rights, told Al Jazeera on Saturday he was getting several reports of family members of those protesting in the US being targeted back home.
"There are reports of intimidation and of retaliation. At this moment we are asking for investigation, but we are not quite certain," he said.
"There are enough reports that I think requires somebody to look into that.
"For instance, we had someone who is very active and calling for democracy in Syria and protesting in front of the embassy in Washington - his family member was killed back in Syria."
The US action comes a day after Syria accused the US ambassador in Damascus, Robert Ford, of inciting protests in the city of Hama and "interfering" in its internal affairs.
"The presence of the US ambassador in Hama without previous permission is obvious proof of a clear evidence of the United States' involvement in current events in Syria," Syria's foreign ministry said in a statement.
The US has rejected the charge, saying that Ford was welcomed with flowers and olive branches by peaceful civilians seeking political change.
"Absolute rubbish," Victoria Nuland, a state department spokesperson, said.
"The reason for his visit was to stand in solidarity with the right of the Syrian people to demonstrate peacefully."
She said the US embassy had informed Syria's defence ministry before Ford's trip and he had passed through Syrian checkpoints along the way.
Diplomats said that the French ambassador, Eric Chevallier, was also in Hama to show support.
France's foreign ministry said in a statement that Chevallier's visit showed the country's concern for the Syrian population.
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The developments on the diplomatic front came as Human Rights Watch (HRW) the New York-based rights group, quoted several defectors from Syrian security forces as saying that they received orders to shoot on protesters to disperse them.
HRW interviewed eight soldiers and four members of the security agencies, who participated in the government crackdown in Daraa, Izraa, Baniyas, Homs, Jisr al-Shughur, Aleppo and Damascus.
"All of the interviewed defectors told Human Rights Watch that their superiors had told them that they were fighting infiltrators [mundaseen], Salafists, and terrorists," a press statement said on Saturday.
Protests in Hama
On the ground, opponents of Bashar al-Assad's rule continue to defy the government's security crackdown. More than 500,000 people flooded through the city of Hama on Friday, according to activists, in what they claim was the single biggest protest yet against the embattled government of al-Assad.
The opposition reported 13 protesters killed, including five deaths in the central city of Homs, two in the capital's commercial neighbourhood Midan and six in the Dumair area, east of Damascus.
Hundreds of thousands also protested last Friday in Hama, prompting mass arrests.
An estimated 1,000 Syrians have fled Hama in fear of another military crackdown on protests calling for Assad to quit and an end to the Baath Party's decades-long grip on power, a rights group said.
Source: Al Jazeera and agencies