Syria's ceasefire increasingly was under threat on Sunday as the government vowed a crackdown on a wave of terrorist attacks and its forces shelled Homs on the day the first U.N. peace monitors were due to enter the country.

An initial team of U.N. ceasefire monitors was due to arrive on Sunday evening and will be deployed on Monday in an effort to keep the peace plan on track, the spokesman for international mediator Kofi Annan said.

They will be joined by two dozen more observers in coming days in line with a U.N. Security Council resolution adopted on Saturday authorising the deployment of up to 30, Ahmad Fawzi said.

Of course we are hoping that the process holds together until the observers get on the ground, Fawzi told Reuters in Geneva.

The Syrian government said it could not be responsible for the safety of the monitors unless it is involved in all steps on the ground, government spokeswoman and presidential adviser Bouthaina Shaaban said.

She also said the number of monitors could rise to 250, but that Syria reserved the right to agree on the nationality of those participating.

On the day the first observers with due in Syria, the city of Homs, one of the hotbeds of opposition to President Bashar al-Assad, was being bombed by government forces at a rate of one shell per minute, activists said. They also reported attacks elsewhere in the country.

Syria said it would stop what it called terrorist groups from continuing criminal acts, state TV quoted a security source as saying, casting further doubt on whether the ceasefire would hold.

(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, the state news agency SANA said.

Since the announcement of an end to military operations, terrorist attacks have increased by dozens, causing a large loss of life, SANA added.

ANNAN TO REPORT TO ARAB LEAGUE

The Arab League, which along with the United Nations backed the negotiations by Annan leading to the declaration of a ceasefire, welcomed the Security Council decision to send in monitors.

The Arab League welcomes this decision as it represents an international will to support the mission of the joint envoy Kofi Annan, Egypt's news agency MENA said quoting deputy Arab League chief Ahmed Ben Helli.

Helli said Annan will report on his mission at an Arab League meeting on Syria on Tuesday in Qatar.

Four days after the ceasefire was meant to come into effect, violence persisted.

Early this morning we saw a helicopter and a spotter plane fly overhead. Ten minutes later, there was heavy shelling, said Walid al-Fares, an activist living in the battered Homs district of Khalidiya.

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.

Rami Abdelrahman, head of the British-based Syrian Observatory for Human Rights, said shells were being fired at a rate of one a minute. He said there had also been overnight clashes in rural Aleppo.

People said they heard explosions and shooting after rebels attacked a police station and then clashed with police, he said.

Syria blames the violence on terrorists seeking to topple President Assad and has repeatedly denied journalists access to the country, making it impossible to independently verify the reports.

Although violence has continued throughout the ceasefire, there has been a significant drop in the daily death toll in fighting which has often killed more than 100 people a day.

On Saturday, 14 people were killed, Abdelrahman said and the state news agency SANA said armed terrorists killed five people in ambushes around the country.

Abu Rabea, an activist in Homs, dismissed the ceasefire and the monitoring mission.

Nothing has changed in Homs, government loyalists on roofs are using heavy machineguns to shoot us and we are being shelled. The only thing that has changed is that Kofi Annan's plan is said to be accepted by the regime and the world believes them.

VIOLENCE AND DIPLOMACY

The Security Council resolution condemned the widespread violations of human rights by the Syrian authorities, as well as any human rights abuses by armed groups.

The text, supported by Russian and China, which had vetoed previous Syria resolutions, included a vague warning to Damascus, saying the council would assess the implementation of this resolution and to consider further steps as appropriate.

U.S. ambassador to the United Nations Susan Rice condemned what she said was Syria's murderous rampage over the last year. Asked if Syrian government shelling of Homs on Saturday was a violation of the ceasefire, Rice said: Absolutely.

The U.N. estimates Assad's forces have killed more than 9,000 people in the uprising. Syrian authorities say foreign-backed militants have killed more than 2,500 soldiers and police.

(Additional reporting by Louis Charbonneau and Michelle Nichols at the United Nations, Stephanie Nebehay in Geneva, John Irish in Paris, Avril Ormsby in London and Yasmine Saleh in Cairo; Writing by Oliver Holmes; Editing by Michael Roddy)