Forces loyal to Yemen's president Ali Abdullah Saleh have used helicopters and MiG fighter jets to attack bases of a rival tribal group, sources have told Al Jazeera.
Hakim Al Masmari, the editor of the Yemen Post, said on Friday the fighting was concentrated in the suburb of Nahm, north east of capital Sanaa, and that the air strikes started after the tribesmen belonging to the Hashid tribe attacked checkpoints around a Republic Guards camp, taking control of one of them.
Members of the Hashid tribe have joined the anti-government protesters in calling for Saleh to step down after he backed out of a deal brokered by the Gulf Co-operation Council (GCC) for him to peacefully step down.
Three months of street protests calling for an end to the 33-year rule of Saleh have brought the country to a near standstill.
"The tribes are very powerful, ruthless and well-armed," Al Masmari said.
He said they started attacking loyalist troops after they were attacked. Al Jazeera could not independently verify the claims as Yemeni authorities have prevented foreign journalists from reporting inside the country.
Al Masmari also said the fighting in the capital had died down but that clashes continued outside the capital and that about 20 people had been killed in areas near the capital.
'Ready for war'
Sheikh Sadiq al-Ahmar, the tribal chief, said on Friday that there was a truce between his fighters and security forces in Sanaa, but he was ready for war if the embattled president wants one.
"There is a truce between us and [President] Ali Abdullah Saleh" for mediation to take place, said Ahmar at a funeral for 30 of his fighters killed in clashes with Saleh's security forces in the capital which began on Monday.
"If the Saleh regime wants a peaceful revolution, we are ready for that. If he chooses war, we will fight him," Ahmar said.
A correspondent for the AFP news agency said tens of thousands of people turned out for the funeral while elsewhere in Sanaa, thousands of pro-Saleh demonstrators gathered briefly for a "Friday of Law and Order" demonstration.
Meanwhile, G8 leaders have condemned violence by Yemeni forces against peaceful protesters and called on Saleh to stick to his commitment to end his rule.
"We condemn the use of violence in response to peaceful protest throughout Yemen," the G8 leaders said on Friday after a two-day summit in France.
"We urge President Saleh to immediately follow through on his commitments and ensure that the legitimate aspirations of the Yemeni people are addressed," the group said in a statement.
Early on Friday, clashes broke out between the tribesmen and Republican Guards in the Al-Fardha area, about 75km northeast of Sanaa, on the road to the city of Maarib.
"Early in the morning, we launched attacks against three Republican Guards checkpoints in Al-Fardha area, and we were able to take control of one of the checkpoints," a tribal source said.
Another tribal source said that 12 tribals were killed in the fighting, which he said began because Republican Guards set up checkpoints on the road to prevent them from travelling to Sanaa to reinforce Ahmar.
A third tribal source said that Yemeni aircraft had bombed the area of the fighting, while a fourth said that the tribal fighters later left the checkpoint and opened the road.
A military source said that General Ali Nasser Gatami, the commander of a nearby Republican Guards camp, was killed in the fighting, and an unknown number of Guards were killed and wounded.