Fresh clashes between Shia rebels and tribesmen backed by Sunni Islamists erupted near the Yemeni capital on Friday, the rebel Ansarullah group said, a day after a government mediator resigned.

The rebels have been advancing from their mountain stronghold in the far north to other Zaidi Shia-majority areas nearer Sanaa in a bid to expand their hoped-for autonomous zone in a future Yemen.

But they have met resistance from tribes loyal to the huge Hashid confederation and their Sunni Islamist allies in the Islah party.

Intermittent clashes pitted "our supporters against hostile elements" north of Arhab, 35 kilometres (20 miles) from the capital and near its international airport, Ansarullah spokesman

Mohamed Abdessalam said, without saying if there had been casualties.

A tribal source also said tensions were high in Arhab district, particularly as government mediator Abdulqader Hilal announced his resignation via Facebook on Thursday night.

Hilal, tasked by President Abdrabuh Mansur Hadi with ending the fighting, has accused Ansarullah of failing to respect the terms of a truce agreed last weekend.

But Abdessalam rejected the charge, saying the mediator should resume his mission after talking to the rebels, who are also known as Huthis.

"Ansarullah is calling for guarantees of a simultaneous withdrawal of both sides from Arhab, the neutrality of the army and the opening of a road linking Omran and Sanaa via Arhab," he said.

Last week, the rebels seized areas in the northern province of Omran, leaving more than 150 people dead and overrunning the home base of the Al-Ahmar clan which heads the Hashid confederation.

Political sources have said the rivals had been seeking to win ground ahead of the demarcation of provinces for the creation of a federal Yemen, as agreed in a national dialogue which concluded in late January.

But the Ansarullah spokesman said his movement, which is based in Saada near the border with Saudi Arabia, was not seeking to expand its influence.

"We do not intend to control Sanaa, where our supporters are already present," he said. But "if hostile forces do not stop their attacks on Arhab, we will expel all of them from the region".