06/01/2012 18:17 GMT

UNITED NATIONS, June 1, 2012 (AFP) - The United Nations said Friday the Syrian government has released 223 prisoners, but stressed that many more must still be freed.

UN monitors on Thursday saw 210 detainees released in Damascus and 13 in the southern city of Deraa, UN deputy spokesman Eduardo del Buey said.

UN reports and rights groups have spoken of between 10,000 and 30,000 prisoners held in often secret detention centers around the country where President Bashar al-Assad has been battling an uprising for 15 months.

Syrian state television said Thursday that 500 detainees involved in the uprising had been freed. "We cannot at this stage verify other reports of releases," Del Buey told reporters.

The UN spokesman said UN Secretary General Ban Ki-moon and UN-Arab League envoy Kofi Annan have repeatedly called for the release of all "arbitrarily held" detainees and "clearly many more must be released."

Major General Robert Mood, head of the UN Supervision Mission in Norway, welcomed the release.

"This is a positive act in these challenging times," Mood said in a statement. "It is an encouraging step towards the implementation of Joint Special Envoy Kofi Annan's six-point plan." Freeing all political prisoners is a key part of the plan.

Mood congratulated the families of those freed and said he "extends his heartfelt thoughts and wishes to those waiting to be united with their loved ones."

Assad's government has announced sporadic releases that diplomats have called "token" gestures by the government.

The Syrian Observatory for Human Rights says tens of thousands of people have been detained and more than 13,000 people killed since March 2011.