Tal al-Mallouhi, a young Syrian blogger sentenced on Monday to five years in jail for passing on secrets to a foreign country, worked for the US spy agency CIA, the foreign ministry said Thursday.

In October, Syria's Al-Watan newspaper reported that Mallouhi, a 19-year-old high school student, was being accused of spying for the US embassy in Egypt.

The charge was denied by Washington, which on Saturday called for her "immediate release" and condemned what it called her "secret trial."

State Department spokesman Philip Crowley said Washington "rejects as baseless allegations of American connections that have resulted in a spurious accusation of espionage."

"We call on the Syrian government to immediately release all its prisoners of conscience and allow its citizens freedom to exercise their universal rights of expression and association without fear of retribution."

Syrian rights groups said in late November that Mallouhi was questioned on November 10 by a state security court and returned to a women's prison in Duma, near Damascus.

The Syrian Observatory for Human Rights, the Syrian League for the Defence of Human Rights and the National Organisation for Human Rights in Syria issued a joint statement of "extreme concern" for Mallouhi.

Mallouhi, granddaughter of a former minister under the late Hafez al-Assad, father of President Bashar al-Assad, was "held incommunicado without charge for nine months," Human Rights Watch (HRW) said in September.

She was first detained in late December 2009, the US-based group said.

Mallouhi's home computer, CDs and books were seized by security services, HRW said, although her blog, which contains poetry and social commentary, focuses on the plight of the Palestinians and does not address Syrian politics.