Fierce fighting erupted around the Sanaa home of dissident tribal chief Sheikh Sadiq al-Ahmar on Tuesday as a truce that ended deadly clashes last week broke down, an AFP correspondent reported.
There were also heavy exchanges between the tribesmen and loyalist troops around the headquarters of the military police and the official Saba news agency, as well as in a major thoroughfare in the Yemeni capital, the correspondent said.
The government of veteran President Ali Abdullah Saleh accused fighters loyal to Ahmar of breaking the truce which the tribal chief announced on Friday.
The defence ministry's 26sep.net news website said that his tribesmen had seized both the headquarters of the ruling General People's Congress and the main offices of the water utility.
But sources close to Ahmar accused Saleh's forces of breaking the truce by firing on his compound in the north of the capital.
A plume of black smoke could be seen billowing into the sky over the area, witnesses said.
There was no immediate word on any casualties. Dozens died in last week's fighting.
Ahmar said on Friday that "there is a truce between us and Ali Abdullah Saleh" for mediation to take place, speaking at a funeral for 30 of his fighters killed in clashes with Saleh's forces in the capital between Monday and Thursday.
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"If the Saleh regime wants a peaceful revolution, we are ready for that. If he chooses war, we will fight him," Ahmar said.
And mediator Sheikh Abdullah Badr al-Din said on Sunday that, following mediation, fighters loyal to Ahmar had begun surrendering government buildings in Sanaa that they seized in fighting with security forces last week.
In March, Ahmar pledged his support for protesters who have been demonstrating since January for the departure from office of Saleh, who has been in power since 1978.
The fighting in Sanaa on Tuesday came after security forces killed 21 protesters as they crushed a four-month-old opposition sit-in in Taez, south of Sanaa.
Security service agents backed by army and Republican Guards stormed the protest against Saleh in the city's Freedom Square, shooting at demonstrators and setting fire to their tents, protesters said on Monday.
"At least 20 protesters have been killed," one protest organiser said.
Another protester was killed when police and Republican Guards opened fire later Monday to prevent dozens of demonstrators from returning to the square, a protester said.
More than 200 demonstrators have been killed since the protests first erupted in late January. Scores more have died in armed clashes between loyalist troops and dissident tribesmen.
In the south, suspected Al-Qaeda militants killed six Yemeni soldiers -- two in Zinjibar on Monday, an army officer said, and four more in an overnight ambush of a military convoy near the city, according to a security official.
A security official said on Sunday that suspected Al-Qaeda gunmen had taken control of most of Zinjibar, the capital of Abyan province, in three days of fighting during which officials and medics said at least 21 people were killed.