Capital braced for tribal conflict
Thousands of tribesmen have threatened to descend on Yemen's capital to join the battle against forces loyal to President Ali Abdullah Saleh as the country slid deeper into an all-out fight for power.
Government forces in Sanaa unleashed some of the heaviest shelling yet against their tribal rivals in a dramatic escalation of the conflict.
For months, youth-led protesters have tried to drive out Mr Saleh peacefully.
But their campaign has been overtaken and transformed into an armed showdown between Yemen's two most powerful families, the president's and the al Ahmar clan.
The al Ahmar family heads the country's strongest tribal confederation, which has vowed to topple Mr Saleh after 33 years in power.
Their nearly two week-old battle in Sanaa raises a dangerous new potential in Yemen: that tribal fighting could spread across the impoverished nation.
Tribes hold deep loyalty among Yemen's 25 million people, and the death of a member can easily draw relatives into a spiral of violence.
On Thursday, tribesman attacked security forces in the city of Taiz, south of the capital, apparently to avenge deaths of protesters there last week or to protect them from new crackdowns.
Mr Saleh's security forces have cracked down hard on the street protesters, killing well over 100 since February, but until now tribal fighters had stayed out of the fray.
Thursday's attack suggests other tribes may see the fighting between Mr Saleh and al Ahmar as a sign it is time to get out their guns as well.