The US State Department on Monday renewed calls for the Syrian authorities to release human rights activist and journalist Mazen Darwish amid concerns for his safety.

State Department spokeswoman Victoria Nuland made the appeal as she highlighted his case as part of a US Free the Press campaign in the run-up to International Press Freedom Day on May 3.

On February 16 Syrian security forces arrested Darwish, president of the Syrian Center for Media and Freedom of Expression.

"The Syrian government has held Darwish incommunicado since security forces raided his offices on February 16th," Nuland told reporters.

"And we are very concerned that he could be the subject of torture, abuse or other inhumane treatment," she said.

"We take this opportunity to call on the Syrian government to release Darwish and other journalists that it has imprisoned, as well as all political prisoners, in keeping with the six-point Annan plan," she said.

The peace plan negotiated by UN and Arab League envoy Kofi Annan calls, among other things, for Syria to withdraw forces from population centers, free political prisoners, grant access to journalists and start a political transition to democracy.

Darwish's group is the only Syrian organization specializing in media monitoring and Internet access, and has an advisory role at the UN Economic and Social Council, or ECOSOC.

Officially shut down by the authorities four years go, he has kept it running without authorisation.