By Oliver Holmes
BEIRUT (Reuters) - U.N. peace monitors are due to start their mission in Syria on Monday to oversee a shaky ceasefire undermined by persistent violence and the shelling of the opposition stronghold of Homs by forces loyal to President Bashar al-Assad.
The ceasefire is part of a broader peace plan brokered by international mediator Kofi Annan, but it looked increasingly under threat throughout the weekend as the government vowed a crackdown on a wave of "terrorist attacks" in Syria.
An advance team of five unarmed monitors arrived in the capital Damascus on Sunday evening, a Reuters witness said.
A Syrian official escorting the team at a Damascus hotel told Reuters that more observers were expected to arrive on Monday, but offered no details. Under the U.N. plan, two dozen more observers are due to enter Syria in coming days.
As the monitors prepared to embark on their mission, violence persisted on the ground.
One activist said the city of Homs, one of the hotbeds of opposition to Assad, was bombarded on Sunday by government forces at a rate of "one shell per minute".
Other activist sources said that six people had been killed on Sunday, and four bodies had been found.
Casting further doubt on whether the ceasefire would hold, Syria said it would stop what it called "terrorist groups" from committing criminal acts, state television reported.
Annan, joint special envoy of the United Nations and Arab League, brokered the six-point peace plan in March as part of international efforts to stop 13 months of violence.
The plan calls for the start of political dialogue, the delivery of humanitarian assistance, the release of prisoners including those involved in peaceful protests, freedom of movement for journalists to work throughout Syria.
The U.N. Security Council authorized the deployment of up to 30 unarmed observers on Saturday in the first resolution on Syria the 15-nation council managed to approve unanimously since the uprising erupted in March 2011.
VIOLENCE IN HOMS
Syria blames the violence on what it says are terrorists seeking to topple Assad. It has denied journalists access to the country, making it impossible to independently verify reports.
The U.N. estimates Assad's forces have killed more than 9,000 people in the uprising. Syria says foreign-backed militants have killed more than 2,500 soldiers and police.
On Sunday, the Syrian state news agency SANA said a "terrorist group" ambushed armed forces in Idlib province, killing a soldier and wounding three others.
"Since the announcement of an end to military operations, terrorist attacks have increased by dozens, causing a large loss of life," SANA added.
"(Security forces), based on their duty to protect civilians and the country, will stop terrorist groups from continuing their criminals acts and the killing of civilians," SANA said.
U.N. Secretary General Ban Ki-moon said he was concerned about the shelling of Homs and urged the Syrian government to refrain from any escalation of violence.
"While we welcome the cessation of violence at this time I warn that the whole world is watching with skeptical eyes whether this will be sustainable," he said. "It is important the Syrian government takes all the measures to keep the cessation of violence."
Annan's spokesman said the mission could be expanded to 250 or more but that would require another resolution.
Syrian government spokeswoman Bouthaina Shaaban said Syria could not be responsible for the safety of the monitors unless it was involved in "all steps on the ground".
On the eve of the mission, Syrian forces pounded Homs, activists said. "Early this morning we saw a helicopter and a spotter plane fly overhead. Ten minutes later, there was heavy shelling," said Walid al-Fares, a local activist.
Activist video footage, reportedly from Khalidiya, shows an explosion shortly after the sound of a missile flying through the air. Another whiz follows, and the cameraman, standing in a nearby building, pans across to show a ball of flames and smoke rising into the air.
(Writing by Alessandra Rizzo Editing by Maria Golovnina)