The Syrian government has not yet removed heavy weapons used to quash anti-regime protests as agreed in a UN ceasefire, UN envoy Kofi Annan's spokesman said Tuesday.

Annan spokesman Ahmad Fawzi said the truce was "extremely fragile" and urged President Bashar al-Assad's government to fully implement its end of the deal.

"This means withdrawal of all heavy armoury from population centres," he told UN broadcaster UNTV.

"They are claiming that this has happened. Satellite imagery however and credible reports show that this has not fully happened, so this is unacceptable."

He said Annan, the UN and Arab League envoy for the crisis, planned to denounce the regime's failure to comply with the ceasefire in an address to a closed session of the UN Security Council later Tuesday.

He also said UN observers on the ground had entered areas like Homs and Hama and found that when they are there "the guns are silent". But when they leave, he said, "credible reports" indicate that fire resumes.

Fawzi's remarks came a day after nearly 60 people were reported killed across Syria despite the cease-fire and the upcoming deployment of 300 UN peace monitors.