The first international observers tasked with monitoring a shaky UN-backed ceasefire in Syria have arrived in Damascus, a United Nations spokesman has said.

The news comes after a Syrian official said the government could not be responsible for the safety of monitors unless it is involved in "all steps on the ground".

Presidential adviser Bouthaina Shaaban also said Syria reserved the right to refuse monitors, whose numbers it said will eventually rise to 250 as agreed with the UN, depending on their nationality.

Earlier UN Secretary General Ban Ki-moon said observers would need complete freedom of movement in Syria to monitor the ceasefire.

The UN Security Council yesterday decided to send an advance team of officials to Syria to examine the effectiveness of a truce between government forces and activists.

He said: "I know that it is a very big country so we will try to have a very effective way of monitoring the situation there.

"We need complete freedom of movement, the security provided by the Syrian authorities. This is very important at this time."

Ms Shaaban said the observers would need to coordinate with the Syrian government in order to ensure their safety.

"The duration of the work of observers and priorities of their movement will be in coordination with the Syrian government because Syria cannot be responsible for the security of these observers unless it coordinates and participates in all steps on the ground," she told reporters.

"Syria has the right ... to agree or not to agree on the nationalities of the observers," she added.

However Syria's state-run SANA news agency reported that the country welcomed the dispatch of international observers to monitor a shaky truce because it "has nothing to hide".

A larger observer mission of about 250 people is planned for by the UN and Mr Ban said he would make firm proposals by Wednesday for such a grouping.

He said: "I will make sure that this advance observer mission will be dispatched as soon as possible and try to make concrete proposals by the 18th of April for an official observer mission.

"That I will discuss with Syrian authorities and I will instruct DPKO (UN Department of Peacekeeping Operations) to take the necessary measures."

Mr Ban also said that international mediator Kofi Annan would be discussing the next steps of his peace plan with Syrian authorities.

"At the same time, the political negotiations should continue in an inclusive way involving all the Syrian people," he said.

Elsewhere, Syrian regime forces are reported to have shelled the rebel Khaldiyeh neighbourhood of Homs this morning.

The Syrian Observatory for Human Rights claimed that troops were firing three shells a minute.

Rebels are also reported to have attacked a police station in Aleppo.

Reports of fighting from Syria cannot be independently verified as the government has barred international journalists and rights groups.

Ban Ki-moon voiced concern over the Syrian regime's shelling of Homs and urged the government to do everything to maintain a fragile ceasefire.