Overnight shelling has taken the death toll from fighting between rival Shi'ite and Sunni Muslim groups in a northern town in Yemen to at least 100 people, a spokesman for one of the groups said on Monday.

Sectarian fighting in the town of Damaj has cast a shadow over national reconciliation efforts in Yemen, a neighbor of oil exporter Saudi Arabia and home to one of al Qaeda's most active wings where Washington is keen to foster stability.

Clashes broke out on Wednesday when Houthi fighters, who control much of Saada province on the border with Saudi Arabia, accused Salafi rivals in Damaj of taking in thousands of foreign fighters to prepare to attack them.

The Salafis say the foreigners are religious students who travel from abroad to study Islamic theology at Dar al-Hadith academy established in the 1980s.

Surour al-Wadi'i, a Salafi spokesman, said the death toll, which according to a Reuters count had reached 58 on Sunday, had risen sharply after heavy shelling on Damaj overnight.

"There were massive Houthi attacks throughout the night against Dar al-Hadith academy and student dormitories," Wadi'i said. "The death toll has risen to at least 100."

The figure was for Sunni Muslims killed in the clashes. The Houthis have issued no figures for casualties on their side, and no officials could be reached on Monday for comment.

Wadi'i said a Red Cross delegation managed to enter Damaj on Monday in a convoy of four vehicles, only to come under fire from Houthi snipers who shot and killed a local translator.

The ICRC could not immediately be reached for comment.

The head of a presidential committee tasked with ending the fighting, Yehia Abuesbaa, said on Sunday Houthi fighters had reneged on a promise to stop attacks after six followers, held by the pro-Salafi Al-Ahmar clan in the adjacent Omran province, had been released.

Abuesbaa said the Houthis blocked his entry to Damaj on Sunday to evacuate some 70 seriously wounded people, demanding the six Houthis be first flown by an army helicopter from Omran to Saada.

The Houthis blockaded Damaj for weeks last year, accusing the Salafis of stockpiling weapons, a charge they deny.

Some Sunnis fear the Houthis want to revive the 1,000-year Zaydi Imamate, whose rulers claimed descent from the Prophet Mohammad. The imamate ended in a 1962 military coup.

Saada province is the base for a long-running Houthi rebellion against the government. Complaining of social, religious and economic discrimination in Yemen, the Houthis fought several battles with government forces between 2004 and 2010, when a truce was announced.

Saudi Arabia was drawn briefly into the conflict in 2010 when rebels crossed into its territory.