Syria's opposition has urged the United Nations to send armed peacekeepers into the country after UN monitors suspended their operations because of raging violence.The exiled Syrian National Council, the country's main opposition group, in a statement issued overnight called on the UN Security Council to adopt a resolution under Chapter VII of the UN Charter to arm the observers.

The call came as Syrian troops reportedly laid siege to several districts of the flashpoint central city of Homs on Sunday, a day after violence cost at least 69 lives nationwide.

"At a time when the regime is committing its worst crimes against the Syrian people, we are surprised by the UN observers' decision to suspend their work, because of what they described as 'an intensification' of violence," the SNC said in a statement obtained by AFP.

The United Nations Supervision Mission in Syria (UNSMIS) halted its operations two months into its three-month mandate on Saturday, blaming intensifying violence.

The observers have been targeted almost daily since deploying in mid-April to monitor a UN-backed but widely flouted ceasefire, and were even likened to "sitting ducks in a shooting gallery" by Susan Rice, the US envoy to the United Nations.

The SNC statement said the UN monitors had failed "to pinpoint the source or type of violence in question."

Suspending the mission, it added, "serves the criminal regime's interests, and denies the Syrian people the little protection they had."

The SNC reiterated its calls for the Security Council to "intervene quickly, and to pass a resolution under Chapter VII (of the UN Charter) to arm the UN monitors, so that they can defend themselves... and ensure that the regime stops killing, while enforcing (UN-Arab League envoy Kofi Annan's) peace plan."

Another four people were killed on Sunday, according to the Britain-based Syrian Observatory for Human Rights.

Speaking to AFP via Skype from the Old City neighbourhood of Homs, activist Abu Bilal said the regime assault on several parts of the city was "suffocating."

"They are shelling us all the time. There's very little food and water, and we're running out of medication."

Abu Bilal reiterated fears expressed by the opposition and rights watchdogs on Saturday that, should regime forces enter the besieged districts, people trapped there "will be massacred."

He added that dozens of civilians in the Old City, in central Homs, were wounded, "and many of them will die if they don't get treatment as we can't get any of the injured out."

Explaining his mission's suspension, Major General Robert Mood spoke of an escalation in fighting and of the risk to his 300-strong UN team, as well as a "lack of willingness" for peace by the warring parties.

"There has been an intensification of armed violence across Syria over the past 10 days," Mood said in a statement on Saturday.

"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," he said.

"In this high risk situation, UNSMIS is suspending its activities," Mood said.

The observers "will not conduct patrols and will stay in their locations until further notice," he said, adding that "engagement with the parties will be restricted."

Mood said the suspension would be reviewed daily, and that "operations will resume when we see the situation fit for us to carry out our mandated activities."

Syria's foreign ministry said it "understands" the decision, stressing that "armed terrorist groups" had been threatening its members.

The United States said the decision marked a "critical juncture" for Syria and that it was discussing with its allies the way ahead for a "political transition" as set out in two UN Security Council resolutions.

"At this critical juncture, we are consulting with our international partners regarding next steps toward a Syrian-led political transition as called for in Security Council Resolutions 2042 and 2043," a White House official said.

Resolutions 2042 and 2043 addressed the observers' deployment under the Annan plan aimed at "facilitating a Syrian-led political transition" that leads to democracy, among other conditions.

British Foreign Secretary William Hague expressed "regret" at the suspension, saying that it "calls into serious question the viability of the UN mission."

His Turkish counterpart Ahmed Davutoglu called for tougher UN action.

"In the event that this observer mission pulls back, there is need for the UN Security Council to immediately do a situation assessment and take a new measure to ensure the humanitarian tragedy does not move onto a next level," he said. (AFP)