Story highlights At least 71 people were killed Monday, an opposition group says

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

The opposition says children were used as human shields in Tafas

The 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 amid violence have proved futile.

At least 71 people were killed Monday, the opposition Syrian Observatory for Human Rights said.

The dead included "entire families trapped under rubble" when regime forces shelled the city of Douma, according to the opposition Local Coordination Committees of Syria.

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

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

It accused "security forces and thugs" of using children in Tafas as human shields on their tanks to prevent possible attacks by the opposition Free Syrian Army.

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Also Monday, the Standard Club, a British marine insurer, said that it had stopped coverage for Russian operator Femco's cargo ship, MV Alaed, amid allegations that it is carrying weapons to the Syrian government.

U.S. officials have said the ship is carrying attack helicopters and munitions from the port of Kaliningrad.

Increased violence in Syria in recent days so exacerbated the already risky situation faced by the approximately 300 unarmed monitors that the United Nations announced Saturday it was suspending the operation.

The LCC issued a statement saying the decision "represents a failure of ... the international community to effectively and responsibly deal with the situation in Syria."

Officials from the Syrian National Council said the suspension indicated it was time for world leaders to take more severe steps.

"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."

In Washington, Sen. John McCain called Monday for just that. "The Syrian opposition needs to know that the United States stands with them and that we are willing to take risks to support them when they need it the most," the ranking member of the Senate Armed Services Committee told an audience at the American Enterprise Institute, a conservative think tank. "Inaction denies us the opportunity to have influence with forces in Syria who will one day inherit the country, ceding that to foreign states that may not always share our values."

McCain took aim at what he said is the Obama administration's toothless policy of calling for Syrian President Bashar al-Assad to step aside.

"To say they are leading from behind is too generous," he said. "That suggests they are leading. They are just behind."

The Republican senator from Arizona accused the administration, "in its desperation," of appearing to place its hopes on persuading the Russian government to push al-Assad from power.

President Barack Obama met with Russian President Vladimir Putin on Monday at the G-20 meeting in Los Cabos, Mexico.

Afterward, the two leaders issued a statement calling for "an immediate cessation of all violence" and expressing support for efforts by U.N. and Arab League joint special envoy Kofi Annan to broker a peace deal.

That elicited derision from McCain.

"It was the kind of statement that you usually hear when there's no concrete agreement," he told CNN. "Also, it's a little weird, Orwellian, to consider their statement supporting Kofi Annan's initiative." McCain described the Annan plan as "a total failure."

In his speech, McCain said any military aid should not include U.S. ground forces, but should include other military might from the United States and its allies, such as European and other Arab countries.

McCain rejected characterizations of the current state of affairs inside Syria as a civil war. "It's not a civil war because all the military strength is on one side, and not the other," he said. "At least we ought to give them a chance to have a fair fight."

The suspension of the monitoring mission is a major blow to Annan's peace plan, which had become a symbol of hope for a country torn by relentless attacks during the 15-month uprising.

The Syrian government has blamed the violence on "armed terrorist groups," the vaguely defined entities it has consistently blamed over the past year.

Syrian opposition groups say more than 13,000 people have been killed since 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.