Syrian security forces have resumed their crackdown on the protest hub of Homs and other cities, a day after authorities in Damascus rejected an Arab League plan calling for U.N. peacekeepers.

Activists said tank fire and artillery shelling hit two rebel-held Sunni Muslim neighborhoods in Homs that have spearheaded the 11-month uprising against President Bashar al-Assad. Opposition groups also reported clashes between government troops and army defectors in the central city of Hama and southern Daraa province.

Rights activists say hundreds have been killed in Homs since the government began bombarding opposition-dominated areas in the city on February 4.

British Foreign Secretary William Hague said Monday the Arab League “could not have sent a clearer message to Syria” when it adopted a resolution calling on member states to provide full support to the Syrian opposition. The European Union also backed the plan and urged the United Nations Security Council to act to stop the violence.

The Arab League asked the Council to authorize a joint Arab-U.N. peacekeeping force to implement a cease-fire in Syria and urged member states to “halt all forms of diplomatic cooperation” with the Syrian government.

Damascus rejected the resolution, saying it reflects the “hysteria” of Arab governments who “failed” to achieve a Security Council resolution authorizing “foreign intervention” in Syria.

Russian Foreign Minister Sergei Lavrov said Monday his country is studying the Arab League initiative, but that a cease-fire would have to take hold in Syria before foreign peacekeepers could be deployed. He also said Russia expects the Arab League to clarify some portions of the proposal.

Russia and China vetoed a Western- and Arab-backed resolution on Syria earlier this month. It would have endorsed an Arab League plan for Mr. Assad to step aside and a unity government to form leading to democratic elections.

The United Nations said last month that violence linked to the uprising had killed more than 5,400 people. U.N. officials stopped updating the death toll in January, saying it was too difficult to obtain information. Rights groups say hundreds more people have been killed since then.