- NEW: The United Nations says Syria poses too much of a risk for observers
- NEW: An escalation in violence is limiting the ability of monitors to do their job
- NEW: The monitors were sent in to ensure compliance to Kofi Annan's peace plan
- NEW: At least 25 people are killed Saturday, opposition activists say
(CNN) -- The United Nations suspended all activities in Syria on Saturday due to the escalating levels of violence, the chief of the global body's mission said.
"There has been an intensification of armed violence across Syria over the past 10 days," said Gen. Robert Mood, who heads the U.N. Supervision Mission in Syria.
"This escalation is limiting our ability to observe, verify, report as well as assist in local dialogue and stability projects -- basically impeding our ability to carry out our mandate."
The situation, Mood said, was too high risk.
Again Saturday, opposition groups reported relentless attacks by government forces that terrorized cities and towns across Syria.
At least 25 people were killed Saturday, according to the opposition Local Coordination Committees of Syria. The government said 27 soldiers and police were buried Saturday, according to the state-run Syrian-Arab News Agency.
CNN could not independently verify reports of bloodshed because of restricted access in the country.
The U.N. monitors, whose numbers gradually rose to about 300, were sent in to ensure that both President Bashar al-Assad and opposition fighters were abiding by a six-point peace plan brokered by special envoy Kofi Annan.
A ceasefire took hold April 12 but only nominally, it turned out.
Violence has soared in recent days with reports of heavy government bombardment of towns and cities and chilling massacres of civilians.
On some occasions, the monitors themselves have come under fire.
"The lack of willingness by the parties to seek a peaceful transition, and the push towards advancing military positions is increasing the losses on both sides: innocent civilians, men, women and children are being killed every day," Mood said.
He said U.N. observers will no longer be conducting patrols and will stay in their locations until further notice.
This suspension will be reviewed on a daily basis, Mood said.
"Operations will resume when we see the situation fit for us to carry out our mandated activities," he said.
U.N. monitors were able to enter the town of Houla a day after opposition activists reported a slaughter there. They also managed to arrive in Qubeir after an alleged massacre there, though they were late again after being turned back at checkpoints.
A few days ago U.N. teams were turned back as they tried to reach al Haffa, which had been under bombardment for eight days. By the time the monitors arrived, al Haffa lay charred and deserted. The monitors said they were overwhelmed by the stench of death.
Syrian opposition groups estimate that between 12,000 and 14,000 people have died in the months of uprising against al-Assad's rule.
Saturday, the Damascus suburb of Douma came under continuous shelling, said the opposition Local Coordination Committees of Syria.
"The city has been under a choking siege by the regime's army," the group said. "Local residents are issuing pleas for help for any sort of intervention to provide medical supplies to treat the wounded."
The group reported "a severe shortage of medical personnel and supplies after continuous shelling over a number of hours" in the city.
Dire medical conditions were also reported in Homs, the anti-government bastion that endured heavy shelling once again Saturday, opposition activists said.
The Syrian government said border guards foiled an infiltration attempt Saturday by an "armed terrorist group" trying to enter Syria's Idlib province from neighboring Turkey, the state news agency said.
Throughout the conflict, al-Assad has blamed the bloodshed on terrorists.
Meanwhile, the United States said it was tracking a Russian military cargo ship en route to Syria that is carrying weapons, ammunition and a small number of Russian troops.
U.S. intelligence believes the Russians are sending the ship to help fortify its naval base in Syria as the situation in country continues to spiral out of control, Pentagon officials said.
