GENEVA - United Nations human rights investigators urged Syria on Tuesday to free a group of at least 16 activists, bloggers and journalists who were arrested by authorities last week, saying they risked being tortured in detention.

"The Syrian authorities should end all acts of harassment against human rights defenders and release all those arbitrarily arrested and detained," the four U.N. human rights experts said in a statement from Geneva.

The U.N. experts said those detained Feb. 16 in a raid on the Syrian Center for Media and Freedom of Expression in Damascus included activist Mazen Darwish and U.S.-born blogger Razan Ghazzawi. They cited reports that the activists were blindfolded and taken to a detention centre run by Syria's military intelligence on the outskirts of Damascus.

"I fear that Mr. Mazen Darwich and other persons arrested may be at serious risk of torture or ill treatment," the U.N.'s torture investigator Juan Mendez said. "I am deeply concerned about their physical and mental well-being."

The appeal was also signed by the U.N.'s free speech investigator Frank La Rue, the global body's expert on human rights defenders Margaret Sekaggya, and the head of a U.N. panel against arbitrary detention El Hadji Malick Sow.

The U.N. estimates that at least 5,400 people, most of them civilians, have been killed in the 11-month uprising against Syria's President Bashar Assad.