Syrian forces were locked in fierce gunfights with rebels in one city and shelled another on Monday, just hours after the first UN peacekeepers arrived to oversee a truce aimed at ending a year of bloodshed.
President Bashar al-Assad's forces killed two civilians in the central city of Hama and were fighting rebels at Idlib in the northwest, while also shelling the flashpoint city of Homs, said the Syrian Observatory for Human Rights.
Since a UN-backed ceasefire came into force at dawn on Thursday, at least 41 people, mostly civilians, have been killed in violence that prompted UN chief Ban Ki-moon to urge Syria to ensure the truce does not collapse.
An advance team of six international observers arrived in Damascus late on Sunday, the United Nations said.
The delegation -- the first of 30 monitors the UN Security Council approved on Saturday -- will set up a headquarters and prepare routines so the mission can verify a cessation of hostilities is holding.
"They've arrived and they will start work (on Monday) morning," UN peacekeeping department spokesman Kieran Dwyer said. "The other monitors in the advance party are still expected in Syria in coming days."
The next 25 would come from missions around the Middle East and Africa "so we can move people quickly and they are experienced in the region," he told AFP.
Their mission is just one part of the six-point peace plan that Assad agreed with UN-Arab League envoy Kofi Annan.
The former UN secretary general wants more than 200 observers to be deployed in Syria, but the Security Council has said there would only be a full mission if the violence halts.
The government news agency SANA said Syria "welcomes" the observer mission, and hoped the monitors would see for themselves the "crimes" committed by "armed terrorist groups."
They face a perilous task, with Western nations doubting the Assad regime's commitment to the ceasefire amid reports his forces have kept battering rebel strongholds and clashed with opposition fighters.
A spike in deadly violence forced the Arab League to end its own Syrian monitoring mission in late January, barely a month after sending them.
On Monday, security forces shot dead the two civilians when they opened fire on a car in Hama, said the Observatory.
Elsewhere, fierce fighting broke out at dawn between forces loyal to Assad and rebels in the city of Idlib, the Britain-based monitoring group said.
Regime forces resumed shelling rebel neighbourhoods of Khaldiyeh and Bayyada in the central city of Homs, a day after at least five civilians were killed there.
The authorities on Sunday charged that rebels had "intensified" attacks on security forces and civilians, warning of a response, as state media published a list of alleged acts of violence.
Security forces "will prevent the terrorist groups from continuing their criminal attacks," said a military official quoted by state media, accusing the rebels of a deliberate escalation to wreck the truce.
Ban voiced concern over the shelling of Homs.
"I am very much concerned about what has happened since yesterday and today," he said. "It is important, absolutely important, that the Syrian government should take all the measures to keep this cessation of violence."
China and Russia, which raised earlier reservations over the text of the peacekeepers resolution and had vetoed past resolutions, backed Saturday's vote at the UN Security Council that approved the monitoring mission.
SANA reported, meanwhile, that Foreign Minister Walid Muallem would visit China at the invitation of his counterpart Yang Jiechi, to discuss Annan's mission.
UN Resolution 2042 approved the sending of 30 unarmed military observers as soon as possible and called on both Syrian government and opposition forces to halt "armed violence in all its forms."
It also urged the government to "implement visibly" all its commitments under Annan's peace plan, including the withdrawal of all troops and heavy guns from cities.
The resolution's passage was welcomed by Syria's main opposition.
"We are ready to act to make the Annan plan a success," the Syrian National Council said in a statement signed by its leader Burhan Ghalioun.
The United Nations says more than 9,000 people have been killed since the uprising began. Monitors say the death toll has topped 10,000.