The first international observers tasked with monitoring a shaky UN-backed ceasefire arrived Sunday in Syria, where regime forces pounded a rebel city and killed five civilians.
"They've arrived and they will start work tomorrow morning," said Kieran Dwyer, a spokesman for the UN peacekeeping department announcing that six observers were now in Syria.
Earlier forces loyal to President Bashar al-Assad killed five civilians in shelling of rebel areas in the flashpoint central city of Homs and clashes with gunmen.
UN chief Ban Ki-moon said he was "very much concerned" at the renewed killings and urged the government to ensure that the ceasefire does not collapse.
State-run news agency SANA said Syria "welcomes" the observer mission, and hoped the monitors will see for themselves the "crimes" committed by "armed terrorist groups."
SANA also reported that Foreign Minister Walid Muallem will visit ally China to discuss the mission of UN-Arab League envoy Kofi Annan who designed the ceasefire.
Regime forces subjected the Khaldiyeh and Bayada neighbourhoods of Homs to their fiercest bombardment since the truce came into force at dawn on Thursday, monitors said.
"The bombardment of Khaldiyeh intensified this morning with an average of three shells a minute," the head of the Syrian Observatory for Human Rights, Rami Abdel Rahman, told AFP.
Three civilians were killed in Homs city, including one in Khaldiyeh, he said, while a fourth was shot dead in central Hama province and the fifth died in the Damascsus suburb of Douma.
With clashes warming up and both sides blaming each other for the violence, Syria insisted that its sovereignty must be respected.
"Syria endorsed the UN observer mission because it has nothing to hide and hopes that these observers will convey the real picture of what is happening on the ground," the official news agency said.
SANA reiterated the regime's repeated accusations that "armed terrorist groups funded and armed by foreign parties" are responsible for the violence in Syria.
Syria accepted observers because its "main request was the monitoring of the terrorists' crimes" and because it was part Annan's six-point plan which Damascus accepted, SANA said.
Thirty-two people, most of them civilians, have been killed since the ceasefire brokered by Annan took effect at dawn Thursday, according to the Observatory.
The death toll is sharply down on pre-ceasefire levels after Syria announced it was halting military operations against the rebels on Thursday.
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 on 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."
The UN chief said he would present on Thursday his proposal to enlarge the UN monitoring mission, which will have 30 unarmed military observers at first, to 250 people.
The six observers who arrived Sunday in Syria are the first of 30 monitors who were approved by the UN Security Council in a vote Saturday. "The other monitors in the advance party are still expected in Syria in coming days," Dwyer added.
On Saturday he told AFP they would come from missions around the Middle East and Africa "so we can move people quickly and they are experienced in the region."
China and Russia, which raised earlier reservations over the text and vetoed past resolutions, backed Saturday's vote, ensuring passage of the first Security Council resolution on Syria since the uprising erupted in March 2011.
State news agency SANA said Muallem would visit at the invitation of his Chinese counterpart Yang Jiechi.
"During the visit, Muallem will discuss means of enhancing bilateral relations and the mission of... Annan," the agency reported giving no date for the two-day visit.
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.
Assad and the opposition must also "guarantee the safety of the advance team without prejudice to its freedom of movement and access," and the "primary responsibility" for observers' safety will rest with the Syrian government.
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.
EU foreign policy chief Catherine Ashton said the resolution "presents an unequivocal call from the international community to the Syrian regime to stop violence against its population and to address urgent humanitarian needs."
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.