Government forces have carried out air strikes against tribal forces opposed to President Ali Abdullah Saleh, tribesmen say.
They said air force planes struck an area in Naham province, outside the capital, Sanaa, leaving an unknown number of dead and injured.
Government and tribal forces have been battling each other for days.
Supporters of the government and the opposition have both called for large demonstrations later on Friday.
President Saleh has faced a wave of popular protests similar to those seen in other Arab countries, as well as a mounting challenge from a rival clan.
The air strikes reported in Naham province, north-east of Sanaa, appear to mark a further escalation in recent fighting.
One tribal leader, Sheik Ali Saif, told the Associated Press news agency that his forces had earlier surrounded a Republican Guard camp in Naham to try to prevent government forces from travelling to the capital to fight tribal gunmen.
President Saleh has repeatedly refused to sign a deal that would see him transfer power.