At least 39 people have been killed in overnight fighting as clashes intensified in the Yemeni capital, Sanaa, doctors and officials say.
Violence escalated after a ceasefire broke down between security forces and fighters loyal to Sheikh Sadiq al-Ahmar, head of a tribal confederation.
Analysts say the conflict threatens to drag Yemen into civil war.
President Ali Abdullah Saleh is resisting international pressure to step down after 33 years in power.
On Tuesday, the US urged him to "to move Yemen forward" by signing a Gulf Co-operation Council-brokered deal that would see him hand over to his deputy in return for an amnesty from prosecution.
At least 350 people have been killed in clashes since protesters began demanding Mr Saleh's immediate resignation in January, inspired by the uprisings in countries such as Tunisia and Egypt.
'Very powerful explosions'
The most recent fighting between supporters of Sheikh Ahmar, who leads the powerful Hashid tribal federation, and security forces loyal to the president began in the capital's northern Hassaba district on Tuesday.
Both sides blamed each other for breaking a ceasefire announced on Friday after a week of clashes during which tribesman seized several government ministries and buildings and more than 115 people were killed.
The defence ministry said tribesman had occupied a building near the presidential palace, a day after they seized control of the headquarters of Mr Saleh's ruling General People's Congress and the interior ministry.
Sources close to Sheikh Ahmar meanwhile said security forces had attacked his compound in al-Hassaba. Artillery fire had caused heavy damage, and electricity and water supplies had been cut, they added.
Residents said there had been fighting in the streets throughout the night in the area and several explosions on Wednesday morning.
"There are very powerful explosions. Sounds like missiles or mortars. May God protect us," one person told the Reuters news agency.
There were also reports that a unit of the elite Presidential Guard, led by one of Mr Saleh's sons, had attacked the headquarters of an army brigade responsible for guarding sensitive government buildings.
Sources said the government suspected the brigade's commander, Brig-Gen Mohammed Khalil, was about to defect to the opposition.
Fighting also spread to other areas of the city, with tribesmen seizing the office of the general prosecutor in north-western Sanaa, a five-storey building in the southern district of Hadda, and near the city's airport.
Medical officials in Sanaa said casualties included fighters from both sides.
Yemen's official news agency, Saba, called the tribesmen "armed gangs" and said they had looted state property from the seized buildings.
On Tuesday, another 12 people were shot dead in the southern city of Taiz after security forces opened fire on anti-government protesters who were attempting to re-establish a protest camp cleared the previous day.
There were also further clashes in the coastal town of Zinjibar where government forces have been battling Islamist militants. Five soldiers were killed in an ambush on Tuesday, officials said.