Clashes on Thursday between forces loyal to Yemeni President Ali Abdullah Saleh and dissident tribesmen in the country's second-largest city Taez killed 13 people, five of them civilians, medical and security officials said.
Witnesses said the fighting erupted before dawn as loyalist troops tried to storm the city centre, a stronghold of armed tribesmen who have pledged support to the protest movement against Saleh's 33-year rule.
Security officials said five Yemeni soldiers were killed in battles and medics from the city's Al-Rawda neighbourhood told AFP eight people, including five civilians, were killed and 30 wounded.
Troop of the loyalist 33rd Brigade fired artillery rounds at several neighbourhoods of Taez, a stronghold of the anti-government protests raging since January, but met stiff resistance, residents said.
The heavily armed tribesmen destroyed one army tank stationed near the city's traffic police headquarters, the witnesses added.
All roads leading into the city were blocked by the fierce fighting that has left outlying districts isolated from the city centre.
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The Yemeni government accuses what it says are "militias from Al-Islah," an Islamist movement that is the main opposition party in parliament, of being behind the unrest and deploying in residential areas across Taez.
On Tuesday, Saleh's forces shelled several neighbourhoods of the city, killing one person and destroying dozens of homes, medics and residents said.
Violence across Yemen has left hundreds dead since the protests erupted. A UN-backed power transfer deal signed by the veteran strongman last month has failed to halt the violence.
An opposition leader was called on to form a caretaker government on Sunday after Saleh announced a pardon for those who "committed errors during the crisis."
The announcement that opposition chief Mohammed Basindawa, a former member of Saleh's ruling party, would form a national unity government to rule until early elections in February, was the clearest sign yet that Saleh had accepted the deal to cede power.
The UN Security Council called on Monday for those behind killings and human rights abuses in Yemen to be "held accountable," as demands grow for Saleh to face trial despite promises of immunity from prosecution for him and his family extended under last month's agreement with the parliamentary opposition.