CAIRO—The Arab League urged the quick deployment of hundreds of United Nations observers in an unprecedented mission aimed at halting the killing of Syrian civilians, as the conflict took another grisly turn on Thursday.

Hundreds of mostly Syrian civilians have been killed since an April 12 cease-fire was supposed to begin, even in the presence of an advanced U.N. monitoring team of 15 observers.

The violence has cast doubt over the widely supported but risky initiative, brokered by Arab League-U.N. envoy Kofi Annan, which aims to pave the way for a negotiated peace between President Bashar al- Assad´s government and the opposition. The U.N.´s plan to send in unarmed observers is unprecedented, as it balances monitors´ safety against an urgent humanitarian situation.

The opposition Syrian National Council, meeting with Nabil Elarabi, the league´s secretary-general, pressed for a quick deployment of 300 U.N. monitors in the hopes they would force the regime to stop its attacks. Mr. Elarabi said he requested the same of U.N. Secretary- General Ban Ki-moon on Thursday.

"The U.N. continues stumbling with regard to dispatching the requested number of observers after the Security Council approved 300 observers," Mr. Elarabi said in a statement.

The U.N. said it expected 30 more observers to arrive by week´s end and 100 more by the end of May. Diplomats say the delays are due to difficulties in finding countries willing to provide monitors, training them, getting funding and receiving Syrian visas.

Meanwhile, Syria´s opposition said Thursday that government forces killed at least 70 people in artillery attacks on a poor neighborhood in Hama. It was the second time this week activists reported the shelling of residential areas in the city in alleged violation of the cease-fire agreement.

One Hama resident, reached by Skype, said rocket fire rained down on a handful of low-rise houses in the Masha´a al-Tayar neighborhood. Other activists reported a barrage of artillery shells and posted video footage of concrete rubble—seemingly collapsed houses.

The government disputed the activists´ account, saying 16 civilians were killed when a terrorist group setting up an explosive device went off, damaging six houses.

State-run news agency Sana also reported other alleged attacks by opposition forces across the country, including on a Syrian Red Crescent ambulance on Wednesday that the agency said killed an aid worker and injured his colleague. One Sana report said a Syrian was shot by his son after the father advised him "to refrain from terrorizing people and sabotaging properties."

Neither account from the government nor the opposition can be verified, due to reporting restrictions in Syria. The gap in narratives isn´t unusual in the 13-month-long uprising that activists say has claimed more than 10,000 lives.

The U.N.´s two permanent monitors in Hama visited the site but Annan spokesman Ahmad Fawzi said the observers had no immediate word on what they saw, according to the Associated Press.

The conflict´s opponents accuse the other of trying to sabotage the mission described by many as the last chance for peace in Syria.

"Both sides are bound to exaggerate with so much at stake," said Louay Hussain, a longtime dissident in Damascus who has twice met with Mr. Annan on his visits to Syria.

Syria´s government says its opponents are plotting the failure of Mr. Annan´s initiative by engaging in armed conflict and then blaming the regime to gain international support in the form of funds or arms for their uprising against President Bashar al-Assad.

Opposition activists say they are under a relentless military onslaught which they do not believe the small number of U.N. monitors can deter, or even objectively observe.

Activists say they fear the U.N. initiative has given Mr. Assad time to decisively crush the opposition. They accuse the government of manipulating the observers in a cat-and-mouse game in which the military withdraws and the returns to fight.

Meanwhile, video footage posted by activists of Masha´a al-Tayar in Hama showed scenes of destruction and chaos. Men cried and pleaded to the camera for help amid concrete rubble. "A plane, a rocket," one man says into the camera. "A five meter hole in the middle of the house."

Other men are shown lifting injured people, described in the video as "survivors of the massacre," out of the rubble.

The Local Coordination Committees, a grassroots activist network, said it has documented the names of 462 people killed since the U.N. observers arrived in Syria on April 16.

In a news conference in Cairo earlier Thursday, Syrian National Council member Abdulbaset Sieda urged the full peacekeeping mission, saying, "the regime cannot understand anything other than strong diplomatic initiatives."

He echoed a warning from France a day earlier that the U.N. Security Council should authorize action against Syria that could be military enforceable under Chapter 7 of the U.N. charter.

The League also said it would call on the Security Council to protect Syrian civilians if Syria´s government is found to be in violation of the Annan plan, though Algeria objected the implicit reference to invoking Chapter 7.

Any Security Council action faces stiff resistance from Syrian ally Russia, who has said having the U.N. observers in Syria has played a positive role.

—Joe Lauria in New York contributed to this article. (Copyright © Dow Jones & Company, Inc.) 04/27/12)