PARIS — International pressure for a harsher line on Syria escalated Thursday, with the president of France calling the Syrian leader a liar, the American secretary of state moving a step closer to endorsing use of military force, and the head of the United Nations accusing the Syrian government of failing to carry out nearly every element of a peace plan that went into effect a week ago.

The tougher tone in the diplomatic push for ways to resolve the 13-month-old Syrian conflict came as new reports of violence in Homs and other hot spots of antigovernment sentiment reinforced a view that the peace plan was in danger of disintegrating. Although an agreement was reached with the Syrian government authorizing the deployment of at least 250 United Nations cease-fire monitors there, differences remained on their freedom of movement, nationalities and total number.

President Nicolas Sarkozy of France set the tone as his government convened a meeting of diplomats from nations in the Friends of Syria coalition seeking to aid the opposition to President Bashar al-Assad. Mr. Sarkozy compared Mr. Assad to the late Libyan leader, Col. Muammar el-Qaddafi. “Bashar al-Assad is lying in a shameful way; he wants to wipe Homs from the map like Qaddafi wanted to wipe Benghazi from the map,” Mr. Sarkozy said on Europe 1 radio. “The solution is the creation of humanitarian corridors so an opposition can exist in Syria.”

Foreign Minister Alain Juppé of France said the Friends of Syria group was discussing contingency plans in case a United Nations cease-fire and peace plan, negotiated by Kofi Annan, the special representative of the United Nations and Arab League, continues to unravel.