Syrian rights activists say a government assault on the central city of Hama has killed at least 34 people, hours before international peace envoy Kofi Annan travels to Damascus to try to salvage his peace plan for Syria.

The activists say Syrian security forces bombarded several neighborhoods of Hama from Sunday into early Monday in retaliation for rebel attacks on government positions. They say the fighting killed soldiers, rebels and at least 13 civilians. The casualties could not be independently confirmed.

Mr. Annan is due to arrive in the Syrian capital later Monday for a visit aimed at pressing government and rebel forces to comply with his plan to end Syria's 15-month conflict. Syrian officials said the former U.N. secretary-general would meet Foreign Minister Walid Moallem on Monday and President Bashar al-Assad on Tuesday.

Ahead of his visit, Mr. Annan accused the Syrian government of using heavy weapons in a rebellious town last Friday, during a massacre of at least 108 people, many of them children. He said the firing of artillery and tank shells at Houla in central Syria was a “flagrant violation” of Syria's commitments to stop using such tactics.

The U.N. Security Council issued a press statement Sunday strongly condemning the Syrian government's bombardment of Houla, where U.N. observers confirmed the killing of dozens of civilians. The 15-nation Council's non-binding statement also condemned the killing of Houla residents “by shooting at close range and by severe physical abuse,” but did not say who was responsible.

Syrian U.N. Ambassador Bashar Ja'afari repeated his government's denial of any role in the Houla killings, blaming them instead on armed terrorist groups Damascus says are behind the rebellion.

Russian Foreign Minister Sergei Lavrov said Monday both sides are to blame for the massacre. Russia is a longtime ally of Syria and has shielded President Assad from U.N. sanctions sought by Western and Arab powers who oppose his 11-year rule.

China also has blocked the U.N. Security Council from imposing sanctions on Syria. The Chinese foreign ministry said it is “deeply shocked” by the Houla killings but stopped short of directly criticizing the Assad government. Beijing called on all sides in the Syrian conflict to implement Mr. Annan's plan for ending the violence “immediately and comprehensively.”

The foreign ministry of Iran, another Assad ally, blamed the massacre on terrorists trying to create chaos and instability in Syria and said the foreign powers backing such attacks are doomed to fail.

In remarks on Sunday, British U.N. Ambassador Mark Lyall Grant said the Houla killings were an “atrocity .. perpetrated by the Syrian government.” He said it is time for the Security Council to discuss next steps for dealing with the Syrian crisis.

Earlier, U.N. Secretary-General Ban Ki-moon sent a letter to the Council saying U.N. observers who viewed the bodies of the Houla victims on Saturday saw wounds from artillery and gunfire. He said the monitors also found artillery and tank shells and fresh tank tracks in the area. The observers reported seeing 108 corpses, including those of 49 children and 34 women.

The only measures approved by the Council on Syria include the backing of international envoy Kofi Annan's peace plan and the deployment of several hundred unarmed U.N. soldiers in the country. The U.N. contingent has been trying to monitor an April truce agreement between government and rebel forces but the conflict has continued largely unabated.

The United Nations says more than 10,000 people have been killed in Syria since the government began its crackdown on dissent in March 2011.