"There is a truce between us and (President) Ali Abdullah Saleh" for mediation to take place, said Ahmar, head of the powerful Hashid federation, at a funeral for 30 of his fighters killed in clashes with Saleh's forces in the capital, which began on Monday and lasted until Thursday.
"If the Saleh regime wants a peaceful revolution, we are ready for that. If he chooses war, we will fight him," Ahmar said.
Tens of thousands of mourners turned out for the funeral, an AFP correspondent said, while elsewhere in Sanaa, thousands of pro-Saleh demonstrators gathered briefly for a "Friday of Law and Order" demonstration.
Early on Friday, clashes broke out between fighters from the Nahm tribe and Republican Guards in Al-Fardha area, about 75 kilometres (45 miles) 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 12 Nahm fighters 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 Yemeni aircraft had bombed the area of the fighting, while a fourth said the tribal fighters later left the checkpoint and opened the road.
A military source said 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 or wounded.
Saleh, who has been in power for 33 years, has since January faced protests calling for his departure.
Fighting erupted between Ahmar loyalists and security forces on Monday, a day after Saleh refused to sign a Gulf Cooperation Council-sponsored accord that would have seen him cede power within 30 days in exchange for immunity from prosecution.
Saleh instead issued fresh demands, including that the opposition sign the deal in his presence, and warned of civil war.
The Yemeni defence ministry's 26sep.net news website said on Thursday that Saleh has ordered the arrest of Ahmar and others involved in the unrest.
Ahmar has accused Saleh of dragging the country into civil war.
The Group of Eight summit on Friday expressed "concern" over the situation in Yemen.
"We condemn the use of violence in response to peaceful protest throughout Yemen," the G8 said in a joint declaration as the summit ended in France, adding that it was "supportive of an inclusive process that should bring about political and social reform in Yemen and lead to a peaceful and orderly transition."
"We urge President Saleh to immediately follow through on his commitments and ensure that the legitimate aspirations of the Yemeni people are addressed."
India's foreign ministry on Friday advised its 14,000 nationals in Yemen to leave because of escalating violence in the impoverished Arabian Peninsula country.
And Qatar's official QNA news agency said that Doha has decided to close its embassy in Sanaa at least temporarily over Saleh's rejection of the GCC transition plan.
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