European Union foreign policy chief Catherine Ashton voiced shock at the use of live rounds against protesters in Taez in a crackdown that the UN human rights office said had already killed more than 50 people since Sunday.
Fierce fighting erupted in the capital Sanaa before dawn between troops loyal to embattled President Ali Abdullah Saleh and fighters loyal to Yemen's most powerful tribal leader Sheikh Sadiq al-Ahmar, an AFP correspondent said.
A plume of dark smoke rose into the sky over Ahmar's compound in north Sanaa, witnesses said. There were also heavy exchanges between his 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 Yemeni capital.
Three of Ahmar's fighters were killed in clashes, a medic said. There was no immediate word of any casualties on the loyalist side.
Saleh's government accused Ahmar's fighters of breaking the truce which the tribal chief announced on Friday after four days of ferocious clashes.
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 loyalist forces of breaking the truce by firing on his compound.
In Taez, south of Sanaa, loyalist security forces shot dead seven anti-government protesters, witnesses said, after 21 were killed as a long-running sit-in in a central square was smashed.
Five were killed in central Taez, witnesses said. Others clashed with police while trying to enter the city, leaving two protesters dead.
Witnesses said security forces were attempting to prevent anyone from gathering in Taez today, firing on those who tried to do so.
Protesters have since January been calling for the departure of Saleh, in power since 1978.
The deaths came after security forces smashed a four- month-long sit-in in Taez, killing 21 protesters.
The UN human rights office put the death toll in Taez since Sunday at more than 50 with hundreds more injured.
"The UN human rights office has received reports... that more than 50 people have been killed since Sunday in Taez by Yemeni army, Republican Guards and other government- affiliated elements who forcibly destroyed the protest camp in Horriya Square using water cannons, bulldozers and live ammunition," it said in a statement released in Geneva.
"Such reprehensible acts of violence and indiscriminate attacks on unarmed civilians by armed security officers must stop immediately," said UN human rights chief Navi Pillay.
The EU foreign policy chief said: "I am shocked and condemn in the strongest terms the use of force and live ammunition against peaceful protesters in the city of Taez.
"The continued repression by the Yemeni regime and grave violations of human rights and international humanitarian law cannot be accepted," Ashton added.
Saleh has been clinging to power despite mounting pressure from the international community to agree his departure from office.