International peace envoy Kofi Annan is in Syria to salvage a peace plan that he admits is not being “implemented comprehensively,” as rights activists report a government assault on the central city of Hama that killed at least 34 people.

After arriving in the Syrian capital, Damascus on Monday, Mr. Annan urged “everyone with a gun” to lay down arms and help resolve Syria's 15-month conflict peacefully. He also said he was “shocked and horrified” by the “tragic” killings of at least 108 people in the rebellious town of Houla on Friday.

“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, every individual with a gun. The six point plan must be implemented comprehensively, and this is not happening to date.''

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

His arrival came after rights activists said security forces bombarded neighborhoods in the flashpoint city 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.

Still, Robert Wood, the head of the U.N. observer mission, said Monday he was seeing “positive signs” in some regions, as his team pushes for dialogue and stability. He said he was looking forward to briefing Annan on what the observers have witnessed so far.

“I look forward to be able to convey my impression of the Syrian people, also to share with him that the suffering of the Syrian people is something that they do not deserve, and we will then have discussions at different levels with different people on whatever we can do to bring this forward in a positive direction, UNSMIS as a mission. I will be advising Kofi Annan, and we will then make a comprehensive assessment and continue our work in that regard.”

The U.N. Security Council issued a press statement Sunday strongly condemning the killing in Houla, where U.N. observers confirmed the deaths of dozens of civilians.

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. Chinese Foreign Ministry spokesman Liu Weimin said Monday that China is “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.”

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.

The 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.

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.