Syrian activists report government forces are pounding a rebel-held section of the central city of Homs, the latest violence to grip a nation already losing its hold on an ever shakier cease-fire.

The British-based Syrian Observatory for Human Rights says Syrian forces are bombarding the Khaldiyeh neighborhood in Homs with mortar rounds, reporting explosions every five minutes. The group also says gunfire is echoing through the streets of Qusair, near the border with Lebanon and that government reinforcements are on their way to help.

Friday's violence is the latest in a series of clashes that have erupted since the United Nations-brokered truce went into effect last week. And in Geneva Friday, a spokesman for international envoy Kofi Annan warned the situation was becoming more dire.

Ahmad Fawzi called the cease-fire “very fragile,” describing the situation as “not good.” Fawzi also expressed hope that the remaining members of a U.N. team of monitors would soon be in place. Meanwhile, concerns that diplomatic efforts and more peacekeepers will fall short are growing.

French Foreign Minister Alain Juppe reiterated Friday the world cannot afford to wait much longer for the cease-fire and peace plan that Annan put forward to take root, warning Syria is on the verge of descending into civil war. China, which has blocked stronger U.N. action against the Syrian regime, said it was ready to do more to uphold the truce – saying it is willing to contribute observers to the U.N. mission.

On Thursday, U.S. Secretary of State Hillary Clinton called for tough U.N. sanctions to be imposed on Syria if it fails to implement the peace plan.

Clinton made the call Thursday at a Paris meeting of top diplomats of more than a dozen nations who call themselves the “Friends of Syria” and support the Syrian opposition revolt against the autocratic rule of President Bashar al-Assad.

Clinton said Washington believes the U.N. Security Council should adopt an arms embargo on Syria and impose travel and financial sanctions on Syrian officials in order to keep Mr. Assad “off balance.” She also said such a resolution should be passed under Chapter Seven of the U.N. Charter, which authorizes measures, including military action.

Russia has twice used its Security Council veto to block Western and Arab proposals for sanctions on the Assad government, a key Russian ally. Clinton said she tried to persuade Russia to drop its objection to sanctions in a meeting earlier Thursday with her Russian counterpart, Sergei Lavrov, in Brussels. She said Lavrov recognized that Syria is “not in a static situation but a deteriorating one.”

A U.N.-backed survey released on Thursday said 230,000 Syrians have been displaced in the country since the uprising began in March 2011.

The United Nations estimates that Syria's crackdown on the revolt has killed more than 9,000 people.