U.N. envoy Kofi Annan called Monday on "every individual with a gun" in Syria to lay down arms, saying he was horrified by a weekend massacre that killed more than 100 people, including women and small children.

Even Syria's staunchest ally Russia joined sweeping international criticism of the mass killings on Friday in the Houla area, saying the government was at least partly to blame for one of the deadliest single events in the 15-month-old uprising against President Bashar Assad's rule.

"I am personally shocked and horrified by the tragic incident in Houla two days ago, which took so many innocent lives, children, women and men," Annan said as he arrived in the Syrian capital Damascus for talks with Assad and other senior officials.

He called on all sides of the conflict to end the bloodshed, saying "this message of peace is not only for the government, but for everyone with a gun."

The killings in Houla, a collection of villages in the central province of Homs, was condemned internationally although world powers differed over whether Assad's forces were exclusively to blame.

Activists from the Houla area said the army pounded the villages with artillery and clashed with local rebels after protests Friday. Some activists said pro-regime thugs later stormed the area, doing the bulk of the killing by gunning down men in the streets and stabbing women and children in their homes.

The Syrian government rejected that narrative and claimed soldiers were attacked in their bases and fought back in self-defense.

Russian Foreign Minister Sergey Lavrov contended in Moscow both the government and the rebels were to blame. Russia has shielded Syria from U.N. resolutions condemning the regime in the past, but has grown increasingly critical of Assad's government in recent months.

"Both sides have obviously had a hand in the deaths of innocent people, including several dozen women and children," Lavrov said. "This area is controlled by the rebels, but it is also surrounded by the government troops."

Lavrov spoke after talks with visiting British Foreign Secretary William Hague in Moscow.

The brutality of the killings came across in amateur videos posted online that showed scores of bodies, many of them young children, in neat rows and covered with blood and deep wounds. A later video showed the bodies, wrapped in white sheets, being placed in a sprawling mass grave.

The massacre in Houla cast fresh doubts on the ability of an international peace plan put forward by Annan to end Syria's crisis.

Syria has strongly denied allegations that its forces carried out the killings, but the U.N. Security Council after an emergency session Sunday "condemned in the strongest possible terms" the killings in Houla. It blamed Syrian forces for shelling residential areas with tanks and artillery.

It also condemned the killings of civilians "by shooting at close range and by severe physical abuse," but avoided saying who was responsible for these attacks.

The council's statement said the "outrageous use of force" against civilians violated international law and Syrian government commitments under previous U.N. resolutions to stop all violence, including the use of heavy weapons in populated areas. It said "those responsible for acts of violence must be held accountable," and asked the U.N. observer mission in Syria and Secretary-General Ban Ki-moon to investigate the attacks and report back to the council.

Britain and France had proposed issuing a media statement condemning the attack on civilians and pointing the finger at the Syrian government. But Russia called for an emergency council meeting, saying it first wanted a briefing by Maj. Gen. Robert Mood, the head of the unarmed U.N. observer mission in Syria.

Mood told the Security Council that U.N. observers at the scene now estimate 108 people were killed in Houla, U.N. peacekeeping chief Herve Ladsous told reporters outside the council chamber.

The U.N. counted 49 children and 34 women among the dead.

Syrian officials denied activists' accounts of the events in Houla and painted a vastly different picture.

Speaking to reporters in Damascus, Foreign Ministry spokesman Jihad Makdissi said Syrian security forces were in their local bases Friday when they were attacked by "hundreds of heavily armed gunmen" firing mortars, heavy machine guns and anti-tank missiles, starting a nine-hour battle that killed three soldiers and wounded 16.

The soldiers fought back, but didn't leave their bases, he said.

"No Syrian tank or artillery entered this place where the massacres were committed," he said. "The security forces did not leave their places because they were in a state of self-defense."

Makdissi did not provide videos or other evidence to support his version of events, nor did he give a death toll. He said the government had formed a committee to investigate and share its findings with Annan during his Damascus visit.

The international rights group Human Rights Watch urged a swift investigation.

"There's no way a Syrian military commission can credibly investigate this horrendous crime when so much evidence suggests pro-government forces were responsible," said Sarah Leah Whitson, Middle East and North Africa director for the New York-based group. "Annan should insist that Syria grant access to the U.N. commission of inquiry to investigate this and other grave crimes."

The United Nations estimates that 9,000 people have been killed since the uprising began in March 2011, but hundreds more have been killed since the U.N. provided that figure.

In the latest violence reported across the country, state-run news agency SANA said four law enforcement agents were killed in a suicide explosion outside the capital.

Earlier, activists said troops shelled several neighborhoods in Hama until the early hours of the day, killing at least 24 people, according to British-based activist groups Syrian Observatory for Human Rights and the Local Coordination Committees. Amateur videos showed a makeshift hospital where several people lay on the floor either dead or wounded