Syrian forces have renewed their assault on the flashpoint city of Homs in what activists say is the heaviest shelling in days, after a warning by the U.N. human rights chief that the onslaught could amount to “crimes against humanity.”

The Local Coordination Committees opposition group Tuesday described Homs as under “brutal shelling.” Witnesses on the ground said food and fuel are scarce and most shops shut due to the relentless mortar and rocket fire that has trapped people in their homes.

The attack has particularly targeted the rebel-held Sunni Muslim district of Baba Amr, an area at the heart of the 11-month anti-government uprising. Internet videos showed black smoke and flames as shells crashed into buildings. Hundreds have been reported killed in Homs since the government began bombarding rebel-held districts.

U.N. High Commissioner for Human Rights Navi Pillay says the Security Council's failure to act has “emboldened” President Bashar al-Assad's government to launch an all-out assault. Syrian officials have blamed “armed terrorists” for the revolt against Mr. Assad.

Syrian state media on Tuesday rejected Pillay's claim that the crackdown amounts to crimes against humanity.

China and Russia have faced a barrage of criticism for vetoing an Arab- and Western-backed U.N. Security Council resolution calling for Mr. Assad to step aside and allow the formation of a unity government to prepare for democratic elections.

Diplomats at the United Nations said a draft General Assembly resolution which supports an Arab League plan calling for the appointment of a joint U.N.-Arab League envoy on Syria could be put to a vote Wednesday or Thursday. There are no vetoes in General Assembly votes and its decision are not legally binding.

The Obama administration said Monday it is in talks with international partners about the league's latest proposal to deploy an international peacekeeping force in Syria.

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

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.