Street fighting raged here in Yemen’s capital on Tuesday after a tenuous cease-fire between tribal groups and forces loyal to President Ali Abdullah Saleh broke down, edging the impoverished Arab state closer to civil war.

Global powers have been pressing Saleh to sign a gulf-mediated deal to hand over power to stem spreading chaos in Yemen, a haven for al-Qaeda militants and neighbor to the world’s biggest oil exporter, Saudi Arabia. The turmoil was a factor keeping oil prices up Tuesday, traders said.

“The cease-fire agreement has ended,” a government official said, adding that tribal groups had seized a government building.

U.N. human rights chief Navi Pillay said her office had received partly confirmed reports that more than 50 people had been killed by Yemeni government forces since Sunday.

On Tuesday, there were three main flash points in the troubled country: street fighting in the capital; government troops gunning down protesters in Taiz, about 120 miles to the south; and a battle against al-Qaeda and other Islamic militants in the coastal city of Zinjibar.

Later Tuesday, heavy explosions rocked a northern district of Sanaa that houses the headquarters of an army division headed by Ali Mohsen, an influential general who has joined the opposition, residents said.

A government spokesman suggested earlier that breakaway army units had attacked the ruling party’s headquarters in Sanaa.

“What was new in today’s clashes is the use of armored vehicles . . . which the Ahmar [tribesmen] don’t have,” Abdu al-Janadi, a deputy information minister, said, referring to the forces of tribal leader Sadiq al-Ahmar.

Sporadic fighting with tribesmen continued in the nearby Hasaba district, where residents took refuge in basements.

Battles in the capital overnight brought an end to a truce between Saleh’s forces and tribesmen brokered during the weekend after more than 115 people were killed in street clashes last week.

Saleh has defied calls from global leaders, elements in his own military and tens of thousands of protesters to end his 33-year rule, which has brought Yemen close to financial ruin.

He has also exasperated his rich Gulf Arab neighbors by three times agreeing to step down and then reneging at the last minute.

Saleh’s forces fired on hundreds of protesters in Taiz who were trying to gather at the focal point of rallies dubbed “Freedom Square,” witnesses said.

At least three people were killed and scores wounded, medical sources said.

Farther south, government troops and locals have been trying to oust al-Qaeda and other Islamist militants from Zinjibar after they seized the town of 20,000 over the weekend.

The United States and Saudi Arabia, both targets of attacks by Yemen-based al-Qaeda in the Arabian Peninsula, are worried that a spreading state of anarchy is emboldening the group.

Opposition leaders have accused Saleh of deliberately allowing Zinjibar, located near a sea lane where about 3 million barrels of oil pass daily, to fall to al-Qaeda to try to show how chaotic Yemen would be without him.