A deadly assault by Shi'ite Houthi rebels on a Salafi Islamic school planted in their mountain heartland could ignite wider sectarian conflict in Yemen, where instability has already helped al Qaeda militants to take root.
The Houthis, who belong to the Zaydi branch of Shi'ite Islam, have bombarded the sprawling Dar al-Hadith seminary in Dammaj village for two weeks, killing at least 100 people.
Late on Tuesday, Al Qaeda in the Arabian Peninsular (AQAP) pledged revenge for the assault.
Political rivalries may have helped to start the violence, but the struggle over a Salafi outpost deep in Houthi territory is also part of a regional contest between Shi'ite Iran and Sunni Saudi Arabia that has been sharpened by the war in Syria.
Zaydis have for years been alarmed by young Sunnis flocking to Dar al-Hadith, in the northern province of Saada, to study ultra-orthodox Salafi doctrines that cast Shi'ites as heretics.
Houthi militants, whose rebellion is fuelled by the accumulated grievances of many Zaydis, dominate Saada after fighting government forces on and off for nearly a decade.
They detest Dar al-Hadith, proclaiming on Oct. 30 that the Salafis had "turned Dammaj into a launch-pad for their criminal actions and a training centre (for) thousands of armed foreign elements from more than 120 countries".
Dar al-Hadith's leaders, who deny any such activities, have condemned al Qaeda, but some militants, including the "American Taliban", John Walker Lindh, have been through the school, and its founder was linked to a 1979 Islamist uprising in Mecca.
The Dammaj fighting is one of many crises besetting Yemen, where state failure could further empower an al Qaeda wing that has targeted Western ships and airliners in the past.
JOCKEYING FOR POSITION
Yemen's best hope may lie in national reconciliation talks begun in March to draft a new constitution and defuse threats from Houthis, southern secessionists and Islamist insurgents.
That effort followed the negotiated removal of veteran President Ali Abdullah Saleh after mass protests against his rule that began even before a wave of Arab revolts in 2011.
Under the deal, interim President Abd-Rabbu Mansour Hadi must hold elections next year and try to reach a workable compromise among Yemen's myriad tribal and political factions.
The Houthis' onslaught on Dammaj may be a gambit to strengthen their bargaining power before any such deal.
"We are coming to the conclusion of the dialogue and each party is trying to consolidate its presence on the ground," said Yemeni analyst Abdul-Ghani al-Iryani. "Dammaj is deep in Zaydi territory. The Houthis are trying to capture it militarily."
Houthi-Salafi strife could further poison the once-relaxed relations between Shi'ites and Sunnis in Yemen, already strained by Iranian-Saudi rivalry and feelings heated by the sectarian hatred that now imbues Syria's struggle between Sunni Islamist rebels and a president whose Alawite sect derives from Shi'ism.
"Yemenis don't hate each other for sectarian reasons," Iryani said. "But that does not preclude this outcome, down the line, if this crisis is not fixed quickly."
Saleh, the former president, exploited sectarian sentiments during his successive wars with the Houthis, according to Ibrahim Sharqieh, a Yemen expert at Doha Brookings think-tank. "This conflict did not start last week," he said.
Zaydis dominate the rugged highlands of Yemen, which their Imams ruled for 1,000 years until a 1962 military coup.
They are well-represented in Yemen's political, military and tribal elites, but have generally kept their faith out of national politics - even as Salafi influence began to rise in mosques funded by Saudi Arabia and other Gulf neighbours.
The Houthis emerged in the north in the 1990s in response to economic deprivation and waning Zaydi political influence, as well as Salafi inroads in the area, symbolised by Dar al-Hadith.
Saleh's military attempts to crush the Houthis from 2004 to 2010 prompted Iran to adopt their cause as fellow-Shi'ites, albeit from two very different branches of the sect, although Tehran denies Saudi accusations that it armed them.
In 2009 Saudi Arabia even fought a brief war with the Houthis, who control territory just over its southern border.
Riyadh's links to Yemen go deep. It has long subsidised the government, as well as funding unruly tribes in a complex quest for influence in its impoverished and more populous neighbour.
Wealthy and often well-connected donors from Saudi Arabia and other Gulf nations have bankrolled Sunni religious centres in Yemen, including Salafi ones such as Dar al-Hadith.
The seminary's founder, Sheikh Muqbil al-Wadi'i, studied in Saudi Arabia in the 1970s after converting from Shi'ism.
He fell in with radical preacher Juhayman al-Otaybi, who led the two-week seizure of Mecca's Grand Mosque in 1979 by militants seeking an Islamist revolution. Otaybi and dozens of his followers were beheaded after the revolt was crushed.
Sheikh Muqbil, who did not take part in the attack but was accused of writing a statement for Otaybi, was deported to Yemen, where he set up Dar al-Hadith. Within a few years it began drawing thousands of Yemeni and foreign students.
According to Mohammed al-Ahmadi, a Yemeni expert on Salafis, up to 7,000, including hundreds from the United States, Canada, Europe and southeast Asia study there, some living with their families on the compound in Dammaj, a complex of white-washed concrete buildings in a lush valley surrounded by mud houses.
Former students describe a monkish lifestyle of memorising the Koran and studying the sayings of the Prophet Mohammad.
The books of medieval theologian Ibn Taymiya, those of the founder of Wahhabism, Sheikh Mohammed ibn Abd al-Wahhab, and of Sheikh Muqbil himself, fill the seminary's library.
While many students pay for their education, donations by Gulf businessmen helped fund Dar al-Hadith, although these were curbed after the Sept. 11, 2011 attacks on U.S. cities.
Gulf backing for Sunnis and Iranian support for Shi'ites may suggest outright sectarian conflict, but Salafis and Houthis represent radical ideologies in Islam's two main wings, not mainstream Sunnis and Shi'ites, and they share some views.
Both advocate a return to the early teachings of Islam guided by sharia law and both call for a restored Caliphate, a single entity ruling the whole Muslim world.
"They may be enemies, but they both reflect the hardline creed of their faiths," Iryani said.