UN mediator Kofi Annan arrived in Damascus today, saying he was "shocked and horrified" by the massacre of 108 people, including 49 children, in the town of Houla.

The architect of the United Nations-Arab League peace plan for Syria said his plan "has to be implemented comprehensively — and this is not happening."

"I have come to Syria at a critical moment in this crisis, Annan said in a statement.

"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.

"This was an appalling crime, and the Security Council has rightly condemned it."

International outrage against Syria intensified Monday, with China and Russia speaking out against the continuing violence.

Annan is scheduled to meet face-to-face with President Bashar al-Assad on Tuesday after a session with Foreign Minister Walid al-Moualem on Monday.

Fresh attacks Monday were reported on the central city of Hama, killing as many as 25, including women and children, even as the Syrian government denied involvement in the weekend massacre.

"Such outrageous use of force against civilian populations constitutes a violation of applicable international law and of the commitments of the Syrian government," the UN said in a statement.

There was little optimism, however, that the Syrian government would curb its attacks on what it views as rebel strongholds.

China condemns 'cruel killings'

The absence of any action by the UN and NATO is "taken by the regime as a carte blanche to do and commit more massacres and atrocities because they know no one is going to come after them," said Tofig Musayev, Azerbaijan's deputy ambassador to the UN, speaking on behalf of the Security Council.

China condemned the "cruel killings" of civilians in the Syrian town of Houla, while insisting that Annan's efforts remained the most viable way to end the violence in Syria.

"China feels deeply shocked by the large number of civilian casualties in Houla, and condemns in the strongest terms the cruel killings of ordinary citizens, especially women and children," Foreign Ministry spokesman Liu Weimin told a daily news briefing.

Russia's foreign minister Sergey Lavrov said after talks with visiting British Foreign Secretary William Hague that the violence must be stopped, but that both sides in the Syrian conflict "had a hand" in the deaths in Houla.

Canada's Foreign Affairs Minister John Baird welcomed a statement by the UN Security Council condemning the Houla massacre.

“This weekend’s shocking attack merits the Security Council’s condemnation and much more," he said. “We call on all Security Council members to come together and adopt strong measures — including economic sanctions — against the Syrian regime, to ensure that it fulfills its commitments and immediately stops the senseless slaughter of its own people.”

The Britain-based Syrian Observatory for Human Rights and the Local Co-ordination Committees say the bombardment of Hama lasted until the early hours of Monday.

The LCC said 24 were killed while the observatory put the death toll at 25, including eight children and five women.

Amateur videos showed a makeshift hospital where several people lay on the floor either dead or wounded.

In 1982, Assad's father and predecessor, Hafez, ordered the military to quell a rebellion by the Muslim Brotherhood movement in Hama in an assault that killed between 10,000 and 25,000 people.

Women and children

The massacre in Houla cast fresh doubts on the ability of Annan's peace plan to end Syria's crisis, which is in its 15th month.

Maj.-Gen. Robert Mood, the head of the unarmed UN observer mission, told the Security Council that UN observers at the scene estimate 108 people were killed in Houla, UN peacekeeping chief Herve Ladsous told reporters outside the council chamber. The UN counted 49 children and 34 women among the dead.

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 Sunday, painting 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, staring a nine-hour battle that killed three soldiers and wounded 16.

'State of self-defence'

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-defence."

He blamed the gunmen for what he called a "terrorist massacre" in Houla and accused the media, Western officials and others of spinning a "tsunami of lies" to justify foreign intervention in Syria.

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, who is due to visit Damascus in the coming days.

Throughout the uprising, the government has deployed snipers, troops and thugs to quash protests and shelled opposition areas.