United Nations-Arab League envoy Kofi Annan said on Wednesday the Syrian government had assured him it will respect a ceasefire with rebel forces, less than 24 hours before a deadline to suspend hostilities.

"I have received government assurances they will respect the ceasefire. If everyone respects it I think by 6 in the morning on Thursday we shall see improved conditions on the ground," Annan said during a visit to Tehran to shore up support for a peace plan backed by the United Nations and Arab League.

Syrian forces pressed home a sustained assault on opponents of President Bashar al-Assad on Tuesday, ignoring an international peace plan under which troops were to silence their guns and withdraw from urban areas.

In a televised news conference, Annan said it was important that regional governments work with Syria to resolve the crisis, adding that Iran could be part of the solution.

Tehran has voiced its support for Annan's six-point plan as long as it does not call for the removal of Syrian president Bashar al-Assad, Iran's closest Arab ally.

Speaking alongside Annan, Iranian foreign minister Ali Akbar Salehi said: "The opportunity must be given to the Syrian government to make changes under the leadership of Bashar al Assad."

The people of Syria should be able to enjoy rights such as freedom of political parties and freedom of elections, but "at the same time we have announced that we oppose interference in the affairs of all nations including Syria," he said.

Rebels did not appear to have stopped fighting either. The anti-Assad Syrian Observatory for Human Rights said insurgents killed six soldiers on Tuesday in attacks on checkpoints on an eastern desert road.

While on a visit to Russia, Syrian Foreign Minister Walid al-Moualem demanded guarantees from Annan that rebels would honor any truce.