International peace envoy Kofi Annan has met with Syrian President Bashar al-Assad in Damascus to urge compliance with a cease-fire torn by clashes between government troops and rebels.

A U.N. spokeswoman said Mr. Annan would raised two main issues in Tuesday's meeting: his six-point peace plan for Syria and the killings of at least 108 civilians in the flashpoint town of Houla last Friday.

Mr. Annan's spokesman said the special envoy “conveyed in frank terms his view to President Assad that the six-point plan cannot succeed without bold steps to stop the violence and release detainees, and stressed the importance of full implementation of the plan.”

Mr. Annan mediated a cease-fire between government and rebel forces last month but it has failed to hold.

In the latest violence on Tuesday, Syrian rights activists said at least 19 people were killed across Syria in attacks related to the uprising.

U.N. spokeswoman Sausan Ghosheh told VOA U.N. observers have remained in Houla since Saturday and have been talking to residents to find details of the Friday incident.

The U.N. human rights office in Geneva said Tuesday the monitors found that fewer than 20 of those killed in Houla were struck by artillery and tank fire. It said witnesses told the observers that most of the other victims were killed in their homes in two waves of summary executions by pro-government militiamen.

The Syrian government denies any role in the Houla killings and blames them on “armed terrorists” whom it accuses of driving a 15-month revolt against Mr. Assad.

After arriving in Damascus on Monday, the former U.N. secretary-general said he was “shocked and horrified” by the Houla massacre, calling it “an appalling crime.”

The U.N. Security Council issued a statement a day earlier blaming some of the deaths directly on “government artillery and tank shellings.” The statement also condemned shootings and stabbings of the other victims but did not say who was responsible.

French President Francois Hollande said Tuesday he is expelling the Syrian ambassador in Paris in response to the massacre. Australia expelled two Syrian diplomats from Canberra in a similar protest. Australian Foreign Minister Bob Carr said he expects other countries will do the same in the coming days.

Speaking on Monday, Mr. Annan appealed to “everyone with a gun” in Syria to resolve the country's conflict peacefully. Government and rebel forces have continued fighting each other despite accepting a truce that he brokered in April.

The U.N.'s Ghosheh said U.N. monitors have not been able to get into a district of Hama city where activists said government forces bombarded residential areas from Sunday into Monday in retaliation for rebel attacks. She said security problems were preventing observers stationed in Hama from entering the district to check on activist reports that more than 34 civilians were killed in the fighting.

The office of British Prime Minister David Cameron said he and French President 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 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.

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 added the international community should first put greater diplomatic pressure on Mr. Assad.

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.