Fighting continues in Yemen between the Houthi rebels and al-Qaeda-backed Sunni tribesmen, leaving at least 68 Houthi fighters dead in the province of Bayda.

The news came as protesters in Yemen's capital Sanaa called on the Houthi fighters to leave after a deadline to form a new government passed on Tuesday without an agreement.

Sporadic clashes erupted between the Houthis and tribesmen in Radaa after the Houthis killed an army officer belonging to the Qaifa tribe, Al Jazeera has learned.

In retaliation, tribesmen reportedly attacked Houthi armed rebels in the northeast of Radaa.

Al-Qaeda fighters are also reportedly in control of the four main areas of Odain district, with the goal of preventing Houthi fighters from advancing in Ibb province in central Yemen.

But the Houthis, members of Yemen's Zaidi Shia community, have made several significant advances recently, taking over the key port city of Hodayda, along with two northern provinces, last week.

The Houthis have seized on chronic instability in the country since the 2012 toppling of long-serving President Ali Abdullah Saleh to take control of large parts of the country.

President Abd-Rabbu Mansour Hadi's Sunni-led central government has so far failed to stop the rebels, despite a UN-brokered peace deal that was supposed to see them withdraw from Sanaa.

'Failed state'

The fighting has raised fears of Yemen - located next to oil producer Saudi Arabia and important shipping routes in the Gulf of Aden - collapsing into a failed state.

Hadi's government is also a key US ally in the fight against al-Qaeda, allowing the US to conduct a longstanding drone war against the group on Yemeni territory.

The Houthis faced no resistance when they took control of Sanaa last month and have refused to leave despite appearing to agree to the naming of a new  prime minister under the UN deal.

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They have since moved south and easily captured Dhamar but have faced heavy fighting in Sunni-majority Ibb province and from al-Qaeda in Bayda.

Tribesmen have accused government forces in the area of collaborating with the rebels in their advance.

Negotiations hosted on Sunday by Yehya al-Iryani, governor of Bayda, to convince all fighters to withdraw from Ibb city "failed", a mediator told AFP news agency.

The Shia rebels have traditionally been concentrated in the northern parts of Yemen, on the border with Sunni-dominated Saudi Arabia.

Yemeni authorities and Saudi Arabia have accused Iran of backing the Houthis in a similar fashion to its support for Lebanon's armed Shia group Hezbollah.

Gulf Arab states have warned that instability in Yemen is threatening regional security.

Security forces on Monday confiscated scores of motorbikes in Sanaa, enforcing a temporary ban aimed at preventing the use of motorcycles in attacks.

Source: Al Jazeera and agencies