Syrian armed forces have kept heavy weapons in cities in breach of a UN-brokered cessation of hostilities, but the government and opposition both have committed truce violations, a top UN official has said.

The 24 unarmed military observers now in Syria have seen howitzer guns, armoured personnel carriers and other weaponry in cities, Herve Ladsous, the UN peacekeeping chief, told a press conference on Tuesday at UN headquarters.

Ladsous insisted, however, that the monitors were having an effect in cities where they have been allowed to go.

Withdrawing weapons and troops from Syrian cities was a key part of a six-point peace plan agreed by President Bashar al-Assad and UN-Arab League envoy Kofi Annan. Syria has told the UN that weapons have been pulled back.

"Regarding the heavy weapons, yes, our military observers do see a number of APCs, for instance, they see a number of Howitzers and other military equipment in most places where they are," Ladsous said.

Syria has told the monitors that the armoured carriers have been disarmed but this has not been verified, Ladsous added.

Ladsous, a UN under-secretary, said that government forces and opposition groups have broken the truce.

"All the parties need to take further steps to ensure a cessation of violence in all its forms," he said.

"The important fact is that violations do come from both sides," he said while refusing to say whether one side had committed more breaches.