Islamist militants who overran a southern town killed five soldiers in an ambush Tuesday while fresh clashes erupted between government forces and fighters loyal to the country's top tribal leader.
The violence pushed Yemen closer to the edge of a civil war.
Nearly four months of mass protests calling for democratic reforms and the ouster of longtime President Ali Abdullah Saleh have rocked the stability of this impoverished corner of the Arabian Peninsula, where government control is weak outside the capital Sanaa and an active al-Qaida branch and other militant groups operate.
Saleh has confronted the mass protests calling for his ouster by promising reform and sending security forces _ his largest remaining bastion of support _ to crack down on protesters. At times, they have unleashed sniper attacks on unarmed marchers.
Saleh has steadfastly refused to step down, clinging to power despite the uprising, defections by key allies and intense pressure from the United States and Yemen's powerful Gulf neighbors to transfer power.
Four protesters were killed in the southern city of Taiz Tuesday, bringing the city's two-day death toll to at least 25.
Stiff criticism of the government's crackdown in Taiz came Tuesday from U.S. State Department spokesman Mark Toner. "We condemn those indiscriminate attacks by Yemeni security forces," he said, urging Saleh to sign an accord to leave office "and to move Yemen forward."
Tuesday's violence highlights the security gaps left open as Saleh's forces work nearly exclusively to keep him in power, said Christopher Bouceck of the Carnegie Endowment for International Peace.
"The more the Yemeni government is focused on this political crisis, the more they're not paying attention to anything else," he said. "So the potential for violence and the breakdown of law and order gets bigger and bigger."
The soldiers were ambushed outside the southern town of Zinjibar, which Islamist militants seized over the weekend. Gunmen fired on an army unit approaching the city from the west, forcing them to accelerate into the fire of other militants hiding down the road, a security official said.
The attack killed five soldier and injured 12, the official said, speaking on condition of anonymity because he was not authorized to speak to reporters. The soldiers killed two militants before fleeing.
Hundreds of armed Islamists stormed the town of more than 20,000 people last week, seizing banks and government offices before setting up barricades to solidify their control. Army units have shelled the town for days, failing to dislodge the militants and while sending hundreds of residents fleeing.
Resident Hilmi Ali said the shells appeared to fall randomly over the town, striking a mosque and four houses in his neighborhood and killing seven of his neighbors.
The Islamists broke into a police administration building and an intelligence office and could be seen speeding about town in police cars, Ali said. Dozens of families fled, braving the gunfights between militants and government forces on the city's outskirts.
"We walked to leave the city," Ali, 21, said by phone from the nearby city of Aden, where many families who fled the fighting are sleeping in schools.
It remains unclear whether the Islamists who seized Zinjibar are connected to Al-Qaida in the Arabian Peninsula _ considered the most active franchise of the terror group in the world. Other armed groups have sought refuge in the area, including some who fought in Iraq and Afghanistan, and others who fought with Saleh's government in a 1994 civil war with the south.
Opposition politicians and army commanders who have abandoned the embattled leader have accused him of allowing the takeover to spread fears of an al-Qaida takeover if his regime falls.
At least 27 soldiers have been killed in the town since Friday. The number of dead civilians and militants remains unclear.
In the capital Sanaa, fresh gunfights and rounds of mortar fire broke out between government troops and fighters from the country's most powerful tribal confederation, shattering a days-old cease-fire.
The fighting brings head-to-head two of Yemen's most powerful men: President Saleh, who has stocked the security forces with relatives and allies his 33-year rule, and Sheik Sadeq al-Ahmar, head of the Hashid tribal confederation.
Tribal fighters seized nine government ministries and other buildings in the downtown Hassaba neighborhood in fighting last week that killed 124 people. They withdrew Sunday from the Local Administration Ministry, a first step in implementing the cease-fire. But they accused Saleh of not withdrawing his forces from buildings around the family compound of al-Ahmar, the Hashid leader.
New clashes erupted early Tuesday and continued throughout the day, with tribal fighters seizing the upper house of parliament, the headquarters of Saleh's ruling party, the Interior Ministry and a key street leading to the airport.
A spokesman for al-Ahmar said the tribal fighters would seize what they needed to remain safe.
"Any place that poses any danger to us and that they are firing at us from, we will take it," Abdel-Qawi al-Qasi said.
Yemeni state media referred to the tribal fighters as "armed gangs" and accused them of looting government offices.
Tribal fighters said at least five of their colleagues were killed Tuesday, and a military statement said one soldier and one civilian were killed.
Government forces shot dead four protesters in the southern city of Taiz Tuesday, medics said, bringing the two-day death toll to at least 25.
Soldiers and tanks cleared a square Monday where anti-Saleh protesters have been camped out for months, bulldozing and setting alight tents and a field hospital established to treat those injured in such attacks.
Tuesday's killing happened when a new protest march was met with gunfire. Protesters set fire to tires and threw stones at the police, who fired rubber bullets, live ammunition and tear gas to disperse them, said activist, Ghazi al-Samie.
"The city is boiling," he said, adding that shops and offices are closed. "And armored military vehicles blocked all the roads leading to the city to prevent people from nearing districts to join the protesters." The U.N.'s human rights office in Geneva said it received reports that government forces had killed more than 50 people in Taiz since Sunday. U.N. High Commissioner for Human Rights Navi Pillay said the reports "remain to be fully verified," but called for a halt to "reprehensible acts of violence and indiscriminate attacks on unarmed civilians."
Hubbard reported from Cairo. Associated Press Writer Bradley Klapper contributed to this report from Washington.