Syria UN observers begin work as fighting continues
The first observers from the UN are starting work in Syria, where they are monitoring a fragile ceasefire.
A UN spokesman told the BBC that the six-strong advance party would make contact with the Syrian government and opposition, before beginning fieldwork.
More observers are expected to arrive later on Monday. Numbers will build up to about 30 by the end of the week.
Violence has continued despite the truce, with about 25 people reportedly dying on Sunday, many of them in Homs.
Activists said security forces had stepped up their shelling of opposition strongholds in the central city. Video footage showed explosions at times at the rate of six a minute in the Khaldiya district.
In the city of Hama, in central Syria, two people were killed by security forces on Monday, opposition activists said.
The claims have not been independently verified.
Under the six-point peace plan put forward by UN-Arab League envoy Kofi Annan, the government in Damascus should have withdrawn its forces and heavy weaponry from cities on 10 April.
UN Secretary General Ban Ki-moon expressed concern, saying the world was "watching with sceptical eyes" to see whether the ceasefire - which came into effect last Thursday - was sustainable.
"It is important - absolutely important that the Syrian government should take all the measures to keep this cessation of violence," he said.
The Syrian government said "terrorist groups" had intensified their "criminal attacks" since the truce began.
The advance party from the UN's Department of Peacekeeping Operations (DPKO) arrived in the capital Damascus on Sunday, a day after the UN Security Council passed a resolution authorising their deployment.
"The first tasks of the six unarmed monitors will be to liaise with the government of Syria, the Syrian security forces and begin to reach out to the opposition so that all sides understand they've arrived, that the monitoring mission pre-advanced team has begun its work in monitoring, and that they all understand that role and respect it," DPKO spokesman Kieran Dwyer told the BBC.
"The Security Council has mandated an advance team of 30, so in the coming days these six will be joined by another 24 military monitors and they will begin to move out to the other cities as quickly as possible."
Mr Dwyer said the Security Council had asked for a report on 18 April to help it determine whether to send a larger observer mission to Syria.
"If any sides are breaching the commitment to end all forms of violence, then there will be international eyes and ears on the ground reporting that," he added.
Syria's state news agency, Sana, said the country welcomed the monitors, and hoped that the team would bear witness to the "crimes" being carried out by "armed terrorist groups".
The BBC's Jim Muir in Beirut says it is unclear whether the observers will get the freedom of movement and access that they need to monitor the ceasefire effectively, and whether they will be able to operate safely.
Under the Security Council resolution covering the mission, the primary responsibility for their safety lies with the Syrian government. But it has said it cannot be responsible if it is not involved in their movements.
Our correspondent says there is clearly a grey area there which needs to be clarified on the ground.
Once the observers do get out and about, one of the first places they are bound to want to go to will be Homs, where troops have used tanks, artillery and rockets to pound areas where rebel fighters are holding out, he adds.
As well as monitoring the truce, the observers are supposed to be supervising the implementation of all aspects of the plan, including the release of all detainees and the provision of aid to stricken areas.
The UN says about 9,000 people have died since pro-democracy protests began in March 2011.
In February, the Syrian government put the death toll at 3,838 - 2,493 civilians and 1,345 security forces personnel.