International peace envoy Kofi Annan is preparing to meet Syrian President Bashar al-Assad in Damascus to urge compliance with a cease-fire that he mediated between government and rebel forces last month.

Mr. Annan was due to hold talks with Mr. Assad on Tuesday, a day after arriving in the Syrian capital and meeting with Foreign Minister Walid Moallam. The U.N.-Arab League envoy began his Syria visit by appealing to “everyone with a gun” to lay down their arms and resolve the country's 15-month conflict peacefully.

The former U.N. secretary-general also said he was “shocked and horrified” by the killings of at least 108 civilians in the rebellious town of Houla last Friday, calling them “an appalling crime.” The U.N. Security Council issued a press statement on Sunday blaming dozens of the deaths directly on “government artillery and tank shellings.” But, the statement did not say who was responsible for shooting and stabbing the other victims.

The Syrian government denies any role in the killings and blames all of them on “armed terrorists” whom it accuses of driving the revolt.

A Vatican statement issued Monday said Pope Benedict was “pained” by the massacre and called on the international community to “spare no effort” to end the Syrian crisis through “dialogue and reconciliation.”

Ahead of Mr. Annan's arrival, activists accused Syrian security forces of bombarding the opposition stronghold 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 34 civilians in the central city. The casualties could not be independently confirmed.

The office of British Prime Minister David Cameron said he and French President Francois Hollande have agreed to increase international pressure on the Assad government to end what they consider to be the “bloody suppression of the Syrian people.” Mr. Cameron and Mr. Hollande discussed the crisis in a Monday phone call.

The two leaders also reiterated their support for Mr. Annan's peace mission in Syria and agreed to work with Syrian ally Russia to find a resolution to the conflict.

Russian Foreign Minister Sergei Lavrov said Monday that evidence indicates Syrian government and rebel forces both are to blame for the Houla massacre. Russia previously has shielded President Assad from U.N. sanctions sought by Western and Arab nations opposed to his 11-year rule, but Moscow recently has toughened its criticism of the Syrian leader's handling of the uprising.

In Washington, the chairman of the Joint Chiefs of Staff, General Martin Dempsey, warned that “atrocities” such as the Houla killings could trigger international military intervention. But, he also stressed that the international community should exert greater diplomatic pressure on Mr. Assad before exploring military options.

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.