International peace envoy Kofi Annan is in Damascus to try to salvage his peace plan for Syria, where rights activists reported a new government assault on the central city of Hama that killed at least 34 people.

After arriving in the Syrian capital on Monday, Mr. Annan repeated his appeal for all sides in Syria's 15-month conflict to end the violence. He also said he was “shocked” by what he called the “tragic” killings of at least 108 people in the rebellious town of Houla on Friday.

Syrian officials said the former U.N. secretary-general would meet Foreign Minister Walid Moallem later in the day and President Bashar al-Assad on Tuesday.

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

On Saturday, Mr. Annan accused the Syrian government of firing artillery and tank shells at Houla the day before in 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.

Lavrov downplayed Russian support for Mr. Assad at a news conference with visiting British Foreign Secretary William Hague. The Russian foreign minister said: “We do not support the Syrian government. We support Kofi Annan's (peace) plan.”

Britain has said the Syrian government is primarily responsible for Syria's violence. But, Hague said London agrees with Moscow on the need to support the Annan peace initiative. He said the alternative is “ever increasing chaos in Syria and a descent … into all-out civil war and collapse.”

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.

Earlier, U.N. Secretary-General Ban Ki-moon sent a letter to the Security 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.

U.N. observer mission chief Robert Mood said Monday he will meet with Mr. Annan to discuss what the Norwegian general called the undeserved suffering of the Syrian people.

The only measures approved by the Security 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.