Yemeni Shi'ite rebels opened fire on government troops sent to quell fighting between the rebels and pro-government tribes, a government official said, threatening a truce in the country's civil war.

Tensions between the rebels and tribes in the country's north have been rising in the Harf Sufyan area for months, and exploded after rebels attacked a tribal leader's home on Friday, killing three of his followers in an explosion.

The Sanaa government, with agreement of the rebels, sent troops to the area on Sunday to restore calm. Rebels said gunfire erupted when the contingent of several hundred soldiers turned out to be much larger than the several dozen they had expected.

"Troops were sent to al-Amshia to secure the Sanaa-Saada road, but the Houthis prevented their arrival and opened fire in their direction," a provincial official told Reuters, referring to the rebels by the tribal name of their leader.

The rebels said they played no role in the shooting, and that civilian gunmen alarmed by the troop influx had opened fire. No injuries were reported, and the troops were prevented  from entering the area.

Yemen, which neighbours top oil exporter Saudi Arabia, agreed a truce with the rebels in February to halt a northern war that has raged on and off since 2004, displaced 350,000 people and briefly drew in Riyadh last year when rebels seized Saudi border areas.

The ceasefire has largely held. But sporadic violence threatens to destabilise further the Arabian peninsula state that is also trying to subdue southern separatism and fight a  resurgent arm of al Qaeda.

Sanaa also accused the rebels of delaying implementing terms of the truce with the rebels, who complain of religious and economic discrimination by the state.

"Despite the halting of military operations ... and the Houthis committing to the truce, they have been avoiding the full implementation," an official, who is part of a committee  overseeing the truce, told the state news agency Saba.

Saba added that the truce committee remained committed to  achieving peace in the north, where last month Yemen accused rebels of killing a government soldier.

The government says rebels have been slow to hand over weapons, including guns, tanks and mortars, and to withdraw from positions in the north. The rebels counter that the state has delayed freeing all rebel prisoners as agreed under the terms of the truce.

Yemen's Western and Saudi allies want Sanaa to resolve its domestic conflicts and consolidate power so that it can focus on fighting al Qaeda, whose Yemeni arm claimed responsibility for a failed attack on a US-bound plane in December.

Previous truces have not lasted and analysts are sceptical that the current ceasefire will hold in the longer term, as long as Shi'ite complaints of discrimination remain unaddressed. – Reuters