Shiite rebels killed 20 people and wounded 70 others in an attack on a Sunni Islamist centre in northern Yemen, a tribal source said on Sunday.
Saturday's attack targeted Dar al-Hadith, a Muslim fundamentalist centre set up in the 1980s to train Sunni preachers in Dammaj, a suburb of the Shiite stronghold city of Saada, said the source.
Dar al-Hadith, which is attended by students from Yemen and many other countries, is seen as a threat by Shiite Huthis who dominate the northern region.
A teacher at Dar al-Hadith told AFP on condition of anonymity that before launching the attack the Huthis had blocked off the Dammaj suburb for two weeks, preventing the delivery of food to about 10,000 people.
According to the teacher, the Huthis want to shut down such religious schools because they fear that Sunnis are trying to convert the Shiites in the region.
The reports could not be immediately confirmed.
Tribal sources say that the Huthis have strengthened their hold on Yemen's northern provinces in recent months due to an anti-regime uprising that has seen troops defect and security forces stretched by popular protests.
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Huthi spokesman Mohammed Abdulsalam said Shiite militants clashed on Saturday with radical Islamists "who have turned Dar al-Hadith into a military camp."
Abdulsalam said the fighting erupted after tribal mediation bid collapsed, adding that less than 10 Huthi militants were killed. The situation was back to normal on Sunday, he said.
Meanwhile a Yemeni security source in the northwestern province of Hijja told AFP that 22 Huthis and six members "from the parliamentary opposition" were killed in clashes over the past three days.
The source said the fighting broke out when gunmen tried to prevent Huthis gaining a foothold in Hijja.
But the Huthi spokesman said "regime militiamen" killed eight Shiite militants in a clash.
Tensions have been running high since President Ali Abdullah Saleh signed a deal with the opposition to step down and hand over his powers to his deputy while a new presidential poll is to take place in February.
The Huthis have denounced the deal, brokered by the six-nation Gulf Cooperation Council, which stipulates immunity for Saleh, accusing the opposition of having "betrayed the blood of the martyrs."
A bloody crackdown on anti-Saleh demonstrations across Yemen since January has left hundreds of people dead.