Syrian rebels killed at least 80 army soldiers at the weekend in a surge of attacks that followed their threat to resume fighting if President Bashar al Assad failed to observe a UN-backed ceasefire, an opposition watchdog said

The Syrian Observatory for Human Rights (SOHR) said doctors had confirmed the deaths of 80 soldiers.

Insurgents told the group they had killed more than 100 soldiers and destroyed tanks in clashes across Syria, including Damascus and the Idlib province in the north-west.

Some commanders in the rebel Free Syrian Army had announced they would be "free of any commitment" to international envoy Kofi Annan's peace plan if Mr Assad did not end violence by Friday.

The May 25 massacre of at least 108 people, nearly half of them children, in the Houla area of Homs province, dealt a possibly fatal blow to Mr Annan's proposed ceasefire, which was supposed to take effect on April 12.

Rami Abdelrahman, the head of the SOHR, said many army checkpoints were destroyed in heavy clashes overnight in Idlib province, where many rebels operate.

He said: "A minimum of four to six in the village of Ariha were attacked and destroyed."

The 15-month-old uprising against Mr Assad's 11-year rule began with peaceful protests, but Syria is now slipping into civil war as rebels fight back against a violent government crackdown.

The UN said Mr Assad's forces have killed more than 9000 people since March, 2011. Syria blames the violence on armed "terrorists" it says have killed more than 2600 soldiers and other members of the security forces.