Army forces loyal to Yemeni President Ali Abdullah Saleh shelled several towns around the southern city of Taiz overnight, an activist said on Monday.
Members of the Republican Guards and the Central Security Forces used heavy artillery against towns on the north-western side of Taiz, the activist who only identified himself as Abdel-Rahim, told the German Press Agency dpa by phone.
He said the towns were home to the armed tribesmen who have pledge to support protesters calling for overthrowing of Saleh's regime.
Millions of Yemenis have been demanding since February that Saleh step down after 32 years in power. More than 350 people have been killed in the government crackdown on protesters, rights groups said.
Saleh is in a Saudi Arabian hospital, recovering from serious injuries sustained during an attack on his presidential palace in June. Vice President Abd-Rabo Mansur is serving as acting president.
At least three tribesmen were killed in the attacks, and several injured, residents said. However, Abdel-Rahim believes the number is higher, but it is hard to know the toll as families prefer to hospitalize their victims at home out of fear that they might be arrested at checkpoints or while in hospital.
More than 200 families were said to have left their towns to escape the attacks, he added.
Clashes between tribesmen and army forces following the shelling left 11 soldiers dead and eight injured, a source at the military hospital in Taiz said. The toll was not verified by security.
The Republican Guards are run by Saleh's eldest son, Ahmed, and the Central Security are led by his nephew, Yehia.
"These military institutions and few others, run by Saleh's family members and confidents are the only obstacle preventing all security forces from being controlled by Mansur," Abdel-Rahim said.
He added that 70 per cent of army forces supported the protesters. Those still backing Saleh were fewer in number but have more advanced weapons and training.
Taiz has seen heavy clashes in the past weeks, especially after security forces stormed the central al-Hurreya square, where protests against Saleh have been going on for months, and set the protest camps on fire. At least 57 people were killed in the attack.