Sanaa, Yemen (CNN) -- A fourth day of bloody clashes between Yemeni security forces and tribal groups left at least five tribesmen dead Tuesday, as a leading tribe commandeered more government buildings in the capital city, according to a spokesman for the leader of the powerful Hashed faction.
Three other people -- who witnesses described as anti-government demonstrators --were killed in the southern city of Taiz, a center of protests against long-time President Ali Abdullah Saleh where at least 50 people have been killed by government forces since Sunday, according to the United Nations and a field hospital staff member.
At least 100 people have been arrested in Taiz, while hundreds more have been injured across the country in recent fighting, the U.N. said Tuesday in a written statement.
"Such reprehensible acts of violence and indiscriminate attacks on unarmed civilians by armed security officers must stop immediately," said U.N. human rights chief Navi Pillay. "I urge all sides to cease the use of force, and I remind the Government of its responsibility to ensure that the fundamental human rights of its citizens are protected."
Pillay noted reports that security forces also had occupied Al-Safa hospital in Taiz, that the field clinic in Horriya Square had been burned, and that there was little or no access to emergency medical care.
Fighting also raged in the southern coastal town of Zinjibar, where an undetermined number of Yemeni soldiers were killed and dozens were injured in fighting over the past two days, the U.N. said.
"Reports of civilian casualties, including children, are particularly worrying, as is the mass displacement of the population of Zinjibar," Pillay said. "The bloodshed must stop."
Meanwhile in Taiz, at least 26 people were injured by gunfire on Tuesday, said Yasser Nomeree, a hospital staffer, and Bushra Maktari, a youth leader.
"Saleh does not want peace," said Abdulqawi al-Qaisi, a spokesman for Sadeq Al-Ahmar, chief of the Hashed tribe, which has opposed government forces in intermittent fighting for more than a month. "Saleh thrives with blood being spilt. They attacked us and we had to defend."
The Organizing Committee of the Youth Revolution said Republican Guards shot at demonstrators in downtown Taiz.
CNN cannot independently verify those accounts.
Meanwhile, fires raged in some areas of Sanaa as clashes took place across the city, according to witnesses.
Ruling Party spokesperson Tareq Shami said mediation efforts, meant to stem the rash of recent violence between the country's tribal groups and Saleh's government, ended Saturday without a peace accord because Hashed tribesmen would not negotiate.
"They occupy ministries and police stations. They walk armed in the streets of Sanaa. They spread fear amongst the people," Shami said. "The tribes are attacking homes of civilians, that is why the ceasefire cannot continue."
Government spokesman Tarek Shami said the al-Ahmar family "started occupying government buildings, therefore the government had to react."
The al-Ahmar family -- which opposes the regime -- captured government buildings in the Hasaba neighborhood of Sanaa, witnesses said, which is also the area where al-Ahmar's home is located.
Hundreds of loud explosions were heard Tuesday in Sanaa as tanks fired mortar shells and heavy artillery fire. Thick black smoke hung in the air as thousands of armed men clashed on city streets.
"The government has ended the cease-fire and started attacking," said Abdulqawi Qaisi, spokesman for the al-Ahmar family. "The tribes of Ahmar need to defend themselves."
The Hashed tribe, which includes the al-Ahmar family, rose up against Saleh this month after the longtime leader backed out of a regionally brokered deal meant to ease him out of office and end months of demonstrations.
Catherine Ashton, foreign policy chief of the European Union, denounced the attacks in Taiz.
"I am shocked and condemn in the strongest terms the use of force and live ammunition against peaceful protesters in the city of Taiz," Ashton said in a statement Tuesday. "The continued repression by the Yemeni regime and grave violations of human rights and international humanitarian law cannot be accepted."
Ashton also called for the United Nations High Commissioner for Human Rights to launch an independent assessment.
U.N. human rights chief Navi Pillay denounced the "intensified use of force against anti-Government protesters," according to a statement from the office of the high commissioner Tuesday.
On Monday, security forces set fire to tents and tore through demonstrators' camps in Freedom Square in Taiz with bulldozers, an activist and eyewitnesses said.
The protest camp was virtually eliminated on Monday, according to Maktati. A field hospital was also dismantled, with the equipment taken away by troops, Maktati, the youth leader, said.
At least 70 tents had been burned down by security forces since late Sunday night, according to witnesses.
On Monday, security forces arrested youths and took them from the streets to an unknown location, Maktati said.
Abdu Ganadi, a government spokesman, said security forces were rescuing colleagues who had been captured and beaten by protesters.
"We did not attack the protesters," Ganadi said. "Reports are all exaggerated. Only two were killed."
He said protesters' tents were burned by people attacked by the protesters, and that tents that burned were empty.
Troops also used water cannons Monday to disperse thousands of protesters in Taiz. A day earlier, clashes left at least 20 people dead and 200 wounded, according to eyewitnesses and two medical officials who could not be named because of security concerns.
Maktati said the attacks would not stop their protests.
"Our revolution will not stop even if hundreds are killed every day," said Sameer Al-Samaee, a leading youth activist in Taiz. "Killing innocent civilians always leads to war crime charges, and that is what we are seeking for Saleh."
The nation's largest cell phone network was ordered to shut down Sunday, according to a senior official with the country's Communications Ministry who requested anonymity because he was not authorized to speak to the media.
The SABAFON network was ordered shut down because of violations and unpaid fines over the last few years, the Communications Ministry official told CNN.
A management official with the SABAFON network, who also was not authorized to speak to the press, confirmed the shutdown. The official denied the government's allegations and said the move appeared to be a tactic to pressure members of the al-Ahmar family, including Hamid al-Ahmar -- the president's chief political enemy.
The official said members of the al-Ahmar family are majority shareholders in SABAFON, with the largest shareholder being Hamid al-Ahmar.
Saleh has resisted protests calling on him to step down after 33 years in power.
The U.S. Embassy in Sanaa has condemned what it called the "unprovoked and unjustified attack" on demonstrators in Taiz. It praised the protesters and called on Saleh "to move immediately on his commitment to transfer power."
The recent fighting has raised fears of a full-blown civil war in Yemen, an impoverished, arid and mountainous nation that has been a key U.S. ally in the battle against the al Qaeda terrorist network.
CNN's Mohammed Jamjoom contributed to this report.