The first UN monitors have arrived in Syria, as part of a peace deal aimed at ending over a year of violence.UN peacekeeping spokesman Kieran Dwyer said the six observers had landed in the capital, Damascus, and would begin work on Monday.
The group left New York after the UN Security Council passed a resolution on Saturday, authorising their deployment.
Earlier, troops reportedly bombarded the restive city of Homs, straining the country's already fragile ceasefire.
The six are the first of 30 monitors approved for the observation mission, although UN and Arab League envoy Kofi Annan, who mediated the plan, hopes to eventually increase the number to over 200.
"The other monitors in the advance party are still expected in Syria in coming days," Mr Dwyer told AFP news agency.
'World is watching' Syria's state news agency Sana said the country "welcomes" the monitors, and hopes that the team will bear witness to the "crimes" being carried out by "armed terrorist groups".
Syria observer mission
- UN resolution 2042 approves a team of up to 30 unarmed observers to deploy to Syria
- An initial group of six monitors arrives in Syria
- The rest of the advance team is set to follow later
- Once certain conditions are met, the UN will seek approval for an expanded force of about 250 observers
- Ban Ki-moon has said he will set out concrete proposals by 18 April for this larger UN observer force
Earlier, speaking from Geneva, UN Secretary General Ban Ki-moon expressed concern after hours of shelling were reported in Homs, and appealed for both sides to abide by the truce.Activists said at least three people were killed in the shelling, which was said to be the fiercest since the truce began at dawn on Thursday. The Syrian Observatory for Human Rights, an activist group, said shells had been fired at a rate of one a minute.
"As I've been stating previously, while we welcome the cessation of violence at this time I warn that the whole world is watching with sceptical eyes whether this will be sustainable," Mr Ban said.
"I'm very concerned what happened yesterday and today when the Syrian government has been shelling the city of Homs and we have already seen some casualties," he added.
Syria said rebels had "intensified" their attacks since the truce began, with a military source on state TV warning that security forces "will prevent the terrorist groups from continuing their criminal attacks".
Our correspondent says the advance team of monitors will look at how to co-ordinate with the Syrian authorities, how freely they will be able to move around and how safe the situation will be.
Although the levels of violence in general are lower than pre-ceasefire Syria, the government has still to conform to one key element of the six-point peace plan that requires the withdrawal of tanks from towns and cities.
Mr Annan has said the UN team would need complete freedom of movement.
But Bouthaina Shaaban, an advisor to President Bashar al-Assad, warned that the Syrian government could not be responsible for the safety of the observer group unless it was involved in "all steps on the ground", Reuters reports.
She also said that Syria had the right to agree to the nationality of those taking part in the mission.
The plan aims to put an end to 13 months of unrest which the UN says has killed at least 9,000 people.