"The president has refused to ratify the ruling of the Supreme State Security Court ... . Therefore the two students were released," Syrian lawyer and human-rights activist Anwar al-Bunni said on Thursday .

"When the sentence is not ratified it becomes void."

The court jailed Muhannad al-Dibis and Muhammad Bashir Arab in February for three years after convicting them of "resisting the goals of the revolution", al-Bunni said.

The students had led protests last year against a decree that ended a tradition of guaranteeing state jobs for all engineering school graduates. The decree was part of an economic and administrative reform programme.

Kurds pardoned

Hundreds of political prisoners have been released since al-Asad came to power in 2000 after his father's death. He has introduced a measure of political freedom and promised wide-ranging reform.

On Wednesday, he pardoned 312 Syrian Kurds accused of taking part in riots.

"These steps are signs that the state is responding in a way in which true (political) reform can be achieved. ... We are waiting for more fundamental reforms," al-Bunni said.

He urged the state to annul a four-decade-old emergency law, order the release of all political prisoners and allow the creation of political parties.

Estimates of the number of political prisoners in the Arab state vary between zero and hundreds, lawyers say.