The official SANA news agency said 265 detainees "who got involved in the recent events in the country, but did not commit murders" were released on May 5, and another 250 detainees were freed May 16.
The announcement came two days after U.N.-Arab League envoy Kofi Annan's latest visit to Damascus to try to save a fragile cease-fire. The Italian news agency ANSA said despite the recent releases many activists and political prisoners remain in Syrian jails.
SANA said 4,482 prisoners "who have not committed" crimes have been released since November.
Wednesday, a Syrian rebel leader said his forces will leave the cease-fire agreement with the government Friday if President Bashar Assad fails to abide by terms of the truce.
In an Internet posting, Free Syrian Army Col. Qassim Saadeddine set a deadline of noon Friday (local time) for Assad to begin carrying out commitments his government made to the six-point peace plan, Voice of America reported.
If Assad fails to comply, Saadeddine said his forces no would no longer be bound by the peace plan, which has been tenuous since it went into effect in mid-April. Despite agreement to the truce, the fighting has continued, with each side accusing the other of breaking the cease-fire.
The Free Syrian Army is a loosely organized and armed rebel group made up mainly of Syrian military defectors.
Susan Rice, U.S. Ambassador to the United Nations, said Wednesday if Annan's plan keeps faltering, the international community must consider new ways to pressure Assad, such as sanctions and taking "actions outside of ... the authority" of the U.N. Security Council.
Rights groups estimate that about 13,000 people have been killed in Assad's crackdown on the pro-democracy uprising since it began March 2011. The Syrian government blames the revolt on foreign-backed terrorists.