Syria opposition: 'Thugs' using kids as shields on tanks

• NEW: Opposition says entire families were killed by regime shelling in Douma
• Civilians are trapped by rising violence in Homs, a U.N. official says
• Opposition officials call for world leaders to consider the use of force
• At least 51 people were killed on Sunday, an opposition group says

(CNN) -- The elusive quest for peace in Syria is now crippled with setbacks, as a U.N. observer mission has suspended operations and attempts to rescue civilians trapped in violence have proved futile.

Dozens of people were killed Monday, including "entire families trapped under rubble" when regime forces shelled the city of Douma, the opposition Local Coordination Committees of Syria said.

"Nobody can rescue the victims because of the continuous shelling," the group said.

Government forces also shelled the southern town of Tafas after more than 80 tanks entered the town, killing three people, the LCC said.

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Violence in the country has escalated in recent days, exacerbating an already risky situation for the approximately 300 monitors, said Gen. Robert Mood, who heads the U.N. monitoring mission in Syria.

"Civilians continue to be trapped by the escalating violence in Syria," Mood said in a statement Sunday. "In Homs, attempts to extract civilians from the line of fire over the past week have been unsuccessful."

He called on government and opposition forces to allow women, children, the elderly and injured victims to leave conflict zones, saying the U.N. mission would monitor their release.

The United Nations announced Saturday it was pulling back its unarmed monitors because mounting attacks in the country were hampering their ability to observe and verify reports. The rising violence has forced observers to stop patrolling and stay put until they are able to resume their mandated activities, Mood said.

Syrian opposition activists slammed the United Nations for its decision, saying it is "unjustifiable and unacceptable" for the international community to fail to protect civilians from attacks.

"The decision of the U.N. supervision mission in Syria to suspend the monitoring mission represents a failure of ... the international community to effectively and responsibly deal with the situation in Syria," the LCC said in a statement Sunday.

Officials from the opposition Syrian National Council said the freezing of the observer mission indicated it was time for world leaders to take more severe steps.

"This mission, unfortunately, has not succeeded. This very small and minimum protection that we were hoping to find with the observers is not there anymore," said Basma Kadmani of the SNC.

SNC officials called on the U.N. Security Council to consider a resolution allowing the use of force in Syria.

"We say that all options are there and must be put on the table," said Abdul Basit Sieda, the group's leader. "This regime only understands the language of violence and force."

The suspension of the monitoring mission is a major blow to a peace plan brokered by Kofi Annan, international special envoy to Syria. The peace plan had become a symbol of hope for civilians under constant attacks during the 15-month uprising.

At least 51 people were killed across Syria on Sunday, including 15 in Homs, opposition activists said.

Some who survive the regime's attacks now suffer from humanitarian crises, Sieda said.

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"The food supplies are very scarce, and medical supplies are not there anymore," he said. "There are wounded people that need to be gotten out of the region, but this criminal regime is still trying to (commit) atrocities."

The Syrian government, meanwhile, has blamed the recent surge in violence -- and the decision to suspend the U.N. mission -- on "armed terrorist groups," the vaguely defined entities it has consistently blamed over the past year.

Twenty members of the army and law enforcement who were killed by such terrorists were laid to rest Sunday, the state-run Syrian Arab News Agency reported.

Syrian opposition groups say more than 13,000 people have been killed since President Bashar al-Assad's government started cracking down on anti-government protesters last year. The United Nations' latest estimate puts the death toll at more than 10,000.

CNN cannot independently verify government and opposition claims of casualties because the Syrian government has restricted access by international journalists.

Journalist Gul Tuysuz contributed to this report.