Medics help an anti-government protester at a hospital after soldiers opened fire Heavy fighting resumed Tuesday in Yemen’s capital between government troops and followers of the country’s most powerful tribal leader, ending a brief cease-fire and again raising the prospect that Yemen’s political crisis could veer into civil war.

Government forces attacked the heavily guarded home of Sheik Sadeq Al-Ahmar, who heads the most powerful tribal confederation in Yemen and has turned against the embattled president to join the protest movement that has been seeking his ouster since early February.

Al-Ahmar’s armed followers fought back and reoccupied several government buildings they had seized in the first round of fighting between the sides last week. There was no immediate information on casualties.

The fighting has dramatically raised the stakes in Yemen’s nearly four-month-old political crisis and further undermined the stability of a deeply impoverished nation home to the most dangerous branch of Al-Qaeda and awash in weapons.

President Ali Abdullah Saleh has clung to power despite daily protests, defections by key allies and intense pressure from the United States and powerful neighbors like Saudi Arabia to transfer his powers.

His security forces have waged a brutal crackdown that has included sniper attacks on unarmed protesters, and he has several times pledged to step down under a deal with the opposition only to back out at the last minute.

Beyond the capital, violence also roiled the southern city of Taiz, which has been a hotbed of anti-government protests since the early days of the uprising against President Saleh, who has ruled Yemen for 33 years.

Soldiers backed by tanks and bulldozers moved in over the weekend, smashing a tent camp the protesters had held in a central square and destroying a field hospital that had been set up in anticipation of such an attack. A doctor who witnessed the attack said at least 20 people were killed on Monday.

On Tuesday, the UN’s human rights office in Geneva said it received reports from Yemen that more than 50 people have been killed by pro-government forces in Taiz since Sunday. UN High Commissioner for Human Rights Navi Pillay cautioned that the reports “remain to be fully verified.” Pillay said in a statement that “such reprehensible acts of violence and indiscriminate attacks on unarmed civilians by armed security officers must stop immediately.” Three more people were killed in clashes in Taiz on Tuesday between security forces and demonstrators who had regrouped on several main streets, said activist Boushra Al-Maqtali.

Protesters set fire to tires and threw stones at the police, who fired rubber bullets, live ammunition and tear gas to disperse them, said another activist, Ghazi Al-Samie.

“The city is boiling,” Al-Samie said. “All shops have been closed and government employees did not go to work and armored military vehicles blocked all the roads leading to the city to prevent people from nearing districts to join the protesters.” In the capital, Sanaa, the fighting between troops and tribesmen centered on the Hassaba district, where the tribal leader, Al-Ahmar, lives.

The overnight shelling and machine gun fire was so intense that many people spent the night in their cellars, said resident Talal Hazza. He said an artillery shell exploded outside his home and another destroyed his neighbor’s house.

He said the fighting prevented ambulances from reaching the wounded.

At least one person was killed — a man who was driving through the neighborhood, said witness Abdel-Waid Ali. But official information on casualties was not available.

Al-Ahmar’s spokesman, Abdel-Qawi Al-Qaisi, said the sheik blamed the government for the renewed fighting, saying it attacked first, damaging his home and several nearby houses.