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At least 39 people have been killed in overnight fighting as clashes intensified in the Yemeni capital, Sanaa, doctors and officials say.
Violence escalated after a ceasefire broke down between security forces and fighters loyal to Sheikh Sadiq al-Ahmar, head of a tribal confederation.
Analysts say the conflict threatens to drag Yemen into civil war.
President Ali Abdullah Saleh is resisting international pressure to step down after 33 years in power.
On Tuesday, the US urged him to "to move Yemen forward" by signing a Gulf Co-operation Council-brokered deal that would see him hand over to his deputy in return for an amnesty from prosecution.
At least 350 people have been killed in clashes since protesters began demanding Mr Saleh's immediate resignation in January, inspired by the uprisings in countries such as Tunisia and Egypt
Source: BBC