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.
The air strikes reported in Naham province, north-east of Sanaa, appear to mark a further escalation in recent fighting.
One tribal leader, Sheikh Ali Saif, told the Associated Press that his forces had taken control of a Republican Guard camp in Naham to try to prevent government forces from travelling to the capital to fight tribal gunmen.
Meanwhile, fighting also flared in the southern city of Zinjibar when dozens of armed militants believed to be linked to al-Qaeda stormed the city, attacking security forces, residents told Reuters news agency.
President Saleh has repeatedly refused to sign a deal that would see him transfer power within a month to a unity government.
It would also give the president immunity from prosecution.
Friday prayers have been a traditional rallying point for protesters calling for Mr Saleh to step down.
Anti-government protesters used Facebook and text messages this week to call for a "Friday of Peaceful Revolution".
One message said the rally "is to stress the peacefulness of the revolution and rejection of efforts to drag the country into a civil war".
Saleh supporters were planning their own "Friday of Law and Order" rally.
A text message said their rally would "condemn the crimes against our rights and the rebellion against the country".
During recent fighting in the capital, hundreds of residents have fled or taken refuge in basements to escape the mortar strikes.
On Friday, the leaders of the G8 group of wealthy nations issued a communique calling on Mr Saleh to stick to his commitment to end his 33-year rule.
"We condemn the use of violence in response to peaceful protest throughout Yemen," the statement read.
"We urge President Saleh to immediately follow through on his commitments and ensure that the legitimate aspirations of the Yemeni people are addressed."