The Obama administration says it summoned Syria's ambassador to the U.S. this week after receiving alarming reports of Syrian diplomats conducting video and photographic surveillance of people participating in protests in the United States.
Authorities may have retaliated against some demonstrators' relatives in Syria.
The State Department said Assistant Secretary for Diplomatic Security Eric Boswell called Syrian Ambassador Imad Mustapha in for discussions Wednesday. Boswell expressed a number of U.S. concerns about alleged actions of Syrian embassy staff.
A statement Friday said the U.S. takes very seriously reports of any foreign government attempting to intimidate individuals in the U.S. exercising their constitutional right to freedom of speech.
It said officials were investigating reports the Syrian government also sought retribution against family members of people seen protesting in the United States.