Officials in Yemen say at least 37 people were killed in heavy fighting overnight in the capital, Sana'a.

Battles raged on several fronts as a tenuous ceasefire announced Friday broke down. The fighting in Sana'a erupted early Tuesday with breakaway army units loyal to Sheik Sadeq al-Ahmar attacking a number of government buildings.

Opposition tribesmen say they took control of the Interior Ministry and the headquarters of President Ali Abdullah Saleh's ruling party, but the government denied the takeover.

Meanwhile, fighting continued Tuesday in two southern cities. Officials say militants killed at least five soldiers in an ambush outside the southern city of Zinjibar, which was seized by hundreds of Islamist fighters on Sunday.

In the flashpoint city of Taiz, also in the south, security forces fired on anti-government demonstrators, killing four people. At least 25 people have died in violence in Taiz in the past three days.

On Monday, Yemeni forces fired live ammunition and crushed a field hospital as they took control of a central square that had been occupied by anti-government demonstrators.

U.N. Human Rights Chief Navi Pillay Tuesday condemned the government's intensified use of force against protesters, calling its acts “reprehensible” and urging the government to make sure the human rights of its citizens are protected.

She also criticized security forces for occupying a hospital in Taiz and destroying the field clinic. Pillay said medical staff and facilities should never be targeted by government forces.

Also Tuesday, an Italian foreign ministry spokesman said Italy has temporarily closed its embassy in Yemen and withdrawn its staff, citing threats against European embassies in certain areas of Sana'a.