World outrage grew Saturday as details emerged about an attack in the Syrian village of Houla, which left more than 90 people dead, including nearly three dozen children, according to the United Nations.

U.N. observers went to Houla and viewed the bodies, a day after opposition activists reported a massacre there at the hands of the Syrian regime. The activists said entire families were killed.

"Whoever started, whoever responded and whoever took part in this deplorable act of violence should be held responsible," Maj. Gen. Robert Mood, the head of the U.N. Supervision Mission in Syria, said in a statement.

Thirty-two children under the age of 10 and more than 60 adults were killed Friday, the statement read. Circumstances that led to the deaths are unclear, he said.

Later, Mood told CNN that observers counted 85 bodies and that 34 of the dead were children under the age of 10. The discrepancy with the earlier number could not immediately be explained.

Observers confirmed the use of artillery and tank shells, Mood said.

"This indiscriminate and disproportionate use of force is unacceptable and unforgivable. The killing of innocent children and civilians needs to stop," said the major general.

He stressed that unarmed U.N. observers are just "one small ingredient" in the effort to end the ongoing violence in Syria.

"At some point, obviously, there will be a discussion about other alternatives and other ways forward, but that is a discussion that has to be within the context of the U.N. Security Council and the key stakeholders to all of this," Mood told CNN.

Syrian state media reported the observers' visits Saturday to several towns and cities and blamed al Qaeda-linked groups for the deaths in Houla.

State TV broadcast Houla images filmed by opposition activists and reported that "more than 50 children were killed ... by criminals and killers who aim to hurt Syria."

The government has consistently blamed "armed terrorists" for violence in Syria.

Rebels leaders interpreted the latest massacre as evidence that a United Nations cease-fire and peace plan aren't working and called for retaliatory attacks.

"We call on our fighters, the soldiers and the revolutionaries, to conduct organized and planned military strikes against Assad battalions and regime members," Brig. Gen. Mustafa Al-Sheikh, a top leader in the rebel group, said in a video statement posted on YouTube.

A network of Syrian opposition activists, the Local Coordination Committees of Syria, blamed "forces and armed militias" of the Syrian government for "a new massacre" in Houla.

"This barbaric act was preceded by the regime's mortar shelling in the town," the LCC said in a statement. "The campaign ended when the armed militias slaughtered entire families in cold blood."

Graphic video posted on YouTube purportedly shows the lifeless bodies of several small children killed in Houla. They are spread on the floor amid blankets, caked in blood. One child is turned to reveal a head wound.

CNN could not independently confirm the authenticity of the video, nor can it confirm reports from within the country because the government strictly limits access by foreign journalists.

In a joint statement with U.N. Secretary-General Ban Ki-moon, joint special envoy to Syria Kofi Annan said he is contacting Syrian authorities "to convey in the clearest terms the expectations of the international community."

"This appalling and brutal crime involving indiscriminate and disproportionate use of force is a flagrant violation of international law and of the commitments of the Syrian government to cease the use of heavy weapons in population centers and violence in all its forms," the joint statement said. "Those responsible for perpetrating this crime must be held to account."

U.S. Secretary of State Hillary Clinton condemned the Houla killings.

"Those who perpetrated this atrocity must be identified and held to account," Clinton said in a statement. "And the United States will work with the international community to intensify our pressure on (Syrian President Bashar al-Assad) and his cronies, whose rule by murder and fear must come to an end."

Lt. Bassim al-Khaled, a spokesman of the rebel Free Syrian Movement, said more bloodshed is forthcoming. The al-Assad government is using the cease-fire and peace plan "to kill more people and is trying to crush the uprising," al-Khaled said.

"So the only language this regime is going to understand is the language of the gun. Wait and see, we will make them pay for each drop of blood which was shed," al-Khaled said.

Britain also condemned the massacre, calling Saturday for an urgent session of the U.N. Security Council and a full account of the "appalling crime."