Yemeni 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.

Since Monday, more than 80 people have been killed in fighting between fighters loyal to Sheikh Sadiq al-Ahmar, head of the Hashid tribal federation, and security forces.