Yemeni opposition tribesmen have taken control of several government buildings in the capital, Sana'a, while engaging in a second day of heavy fighting with forces loyal to President Ali Abdullah Saleh.
Witnesses say tribal fighters allied with Sheik Sadeq al-Ahmar sealed off the commerce ministry and other government offices in northern Sana'a's Hassaba district on Tuesday.
Tribesmen said government forces stationed near the interior ministry fired mortars at al-Ahmar's residence in Hassaba, killing five fighters.
The government accused the tribesmen of attacking a nearby school. At least 12 people, including one government security force member, have been killed in the fighting in recent days.
Al-Ahmar joined unarmed Yemeni opposition activists in March to call for the ouster of Saleh, who also belongs to the Hashid tribe.
The tribal chief's decision to engage his fighters in battle with the government marks an escalation of the near-four month old uprising.
Gulf Arab states suspended their efforts to mediate in the uprising on Sunday, after Saleh refused to sign a deal to hand power to a deputy within 30 days in return for legal immunity.
Saleh backed out of the agreement Sunday after Yemen's opposition signed it the previous day. It was the third time the Yemeni leader has refused to sign the deal.