UN-Arab envoy Kofi Annan arrived in the Syrian capital Monday for a bid to salvage his battered peace plan, expressing "shock" at the massacre of more than 100 people in the town of Houla.

Annan said the "tragic" massacre in the central Syrian town was "an appalling moment with profound consequences," in remarks to reporters ahead of a meeting with Syrian Foreign Minister Walid Muallem.

The former UN secretary general said those responsible for the massacre must be held accountable, and called on "everyone with a gun" to abide by his six-point blueprint to help end 15 months of bloodshed.

World leaders have voiced outrage over the deaths of at least 108 people in Houla on Friday and Saturday, among them 49 children and 34 women, many gruesomely blown to bits or shot dead at point blank range.

The UN Security Council -- where Syrian allies Russia and China wield veto powers -- on Sunday condemned the Damascus government's use of heavy artillery in the assault on Houla.

"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 told reporters at a Damascus hotel.

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

"Our goal is to stop this suffering. It must end and it must end now.

"I urge the government to take bold steps to signal that it is serious in its intention to resolve this crisis peacefully, and for everyone involved to help create the right context for a credible political process. And this message of peace is not only for the government, but for everyone with a gun.

"The six-point plan has to be implemented comprehensively. And this is not happening.

"I intend to have serious and frank discussions with President Bashar al-Assad. I also look forward to speaking with a range of other people while I am here."

Annan's peace plan was supposed to begin with a ceasefire from April 12 but it has been broken daily, and 87 people were killed on Sunday in one of the deadliest days since its nominal start, a watchdog said.

Thirty-four of the dead were killed in random shelling of the central city of Hama by troops retaliating for losses suffered in clashes with rebel fighters, said the Syrian Observatory for Human Rights.

"Hama is like a ghost city," an activist on the spot told AFP by telephone on Monday.

"We are very afraid now, because the regime troops are surrounding the areas where there was fighting, and we fear there might be a new attack," the activist said.

The Observatory, which says more than 13,000 people have been killed in Syria since an anti-regime revolt broke out in March 2011, said there was no excuse for the shelling of civilian neighbourhoods.

The UN Security Council's condemnation of the Syrian government's role in the Houla massacre did little to bring the international powers together to end the crisis.

Britain and France had proposed a text making an even stronger condemnation of the Assad government. But Russia would not agree on the wording and demanded a special meeting before approving the eventual text.

Syria's UN envoy Bashar Jaafari said accusations of government responsibility were part of a "tsunami of lies" against Damascus.

Russia defended its key Middle East ally at the Security Council, and on Monday said both sides in the conflict were responsible for the massacre.

"Here we have a situation where both sides clearly had a hand in the fact that peaceful citizens were killed," said Foreign Minister Sergei Lavrov, adding "who is in power" in Syria was less important than "ending the violence."

Beijing condemned the massacre and called for an "immediate investigation into this issue and to find the perpetrators."

Syria's Muslim Brotherhood lashed out at the Security Council for sending what it said was the "wrong message" to Damascus by only condemning the Houla massacre.

Human Rights Watch demanded that Annan push Assad's government to allow the UN-appointed Commission of Inquiry on Syria to investigate the Houla massacre.

"Annan should insist that Syria grant access to the UN commission of inquiry to investigate this and other grave crimes," HRW's Sarah Leah Whitson said.

Syria's opposition said the massacre was another reason why it should not respect the ceasefire that Annan brokered but which has never taken hold.

The Syrian National Council called for a "battle of liberation and dignity" against the regime until the United Nations allows an international military intervention.

And the Free Syrian Army warned unless the international community took concrete action it would no longer be bound by Annan's plan.

A growing concern for the international community is the more than 280 UN observers now deployed in Syria as part of the Annan plan.

UN leader Ban Ki-moon said the Houla massacre had added to pressure on the monitors -- the first UN force to be thrown unarmed into a conflict with a non-existent ceasefire.

The UN force is "in a perilous position," Ban said, adding there was no "Plan B" if the Annan plan crashes, however.

[UN-Arab envoy Kofi Annan, seen here on May 8, is heading to Damascus in a bid to salvage his battered peace plan a day after the UN condemned the Syrian regime's use of artillery in a massacre that killed more than 100 people. AFP Photo/Violaine Martin]