An advance into Yemen's Sunni Muslim heartland by Shi'ite Houthi fighters has galvanised support for al Qaeda among some Sunnis, deepening the religious hue of the country's many conflicts, with potential consequences well beyond its borders.

Yemen's tribal, regional and political divisions were widened by the rapid fall of the capital Sanaa to Houthi fighters on September 21 after weeks of protests against the government and its decision to cut fuel subsidies.

"The Houthi expansion has created a sectarian problem," said Bassam al-Barq, a Sunni Muslim resident of the religiously mixed Sanaa, attending a protest by local activists held every week to demand the Houthis quit the capital.

"It has created sympathy with al Qaeda, as we see in Ibb and al-Baydah," Barq said, referring to two provinces in central Yemen where some local tribes have allied themselves with al Qaeda's local wing, Ansar al-Sharia.

Ahmed al-Kalaz, a former Yemeni diplomat who comes from al-Baydah, agreed: "The expansion of the Houthis to al-Baydah has created a suitable environment for al Qaeda."

Sectarian bloodshed has spread across the Middle East.

Now Yemen, ancestral homeland of the Saudi-born al Qaeda founder Osama bin Laden, looks to be edging closer to civil war, something that could destabilise its neighbours, including the world's top oil exporter, Saudi Arabia.

But while the Yemen crisis bears some hallmarks of a proxy war between the Gulf's two main powers -- Sunni Saudi Arabia and Shi'ite Iran -- its causes are rooted in local problems.

The country is still struggling to adapt after former president Ali Abdullah Saleh was ousted in 2012 under a U.N. -backed Gulf initiative aimed at preventing 'Arab Spring' protests against him descending into civil war.

Washington suspects the 72-year-old Saleh, who still has supporters in the armed forces, is himself one of the main obstacles to a peaceful transition of power.

"Yemen is seeing a kind of unravelling and violent struggles in a way that is unprecedented," said Abdelbari Taher, a Yemeni historian and political commentator.

"This is due to the volatility of the state, the loss of authority and the splits in the army which made the armed militia stronger than the state."

At stake is not only a Yemeni tradition of religious harmony, but also the fight against al Qaeda, an entrenched force in Yemen with sympathisers in other Gulf Arab states.

Many Sunnis fear the Houthis, who belong to the Zaydi branch of Shi'ite Islam, aim to re-establish the Zaydi imamate which ruled for 1,000 years until a 1962 military coup.

The Houthis, officially known as Ansarullah, deny any such goal. But Sunnis worry their rise could elevate the fifth of Yemenis who follow Zaydism over their Sunni compatriots, despite similarities between Zaydism and the Shafi'i Sunni theology dominant in Yemen.

A further concern is that the strife could cause Yemen to unravel as a state. The inability of state forces to check the Houthis' ascent or dampen sectarianism has galvanised separatist groups who spot an opportunity to push their own agendas.

"When struggles intensify, national identity starts to unravel and everyone returns to his tribe or the group he belongs to," one government official said. "Then it becomes very difficult to reconstitute this issue. Look how long it took Lebanon to restore the Lebanese national identity."

Sectarianism is visible in Radda, a district of al-Baydah province 160 km (100 miles) southeast of Sanaa. Sunni tribesmen who fought al Qaeda two years ago are now allied to it out of communal solidarity.

Al-Baydah, in Yemen's Sunni heartland, has seen heavy fighting between powerful Sunni tribesmen allied with al Qaeda and the Houthis who advanced there in mid-October, after a suicide bomber killed 47 people, most of them Houthis, in Sanna.

The violence bodes ill for Yemen: Al Qaeda in the Arabian Peninsula (AQAP) and its Yemeni arm Ansar al-Sharia has often proved a tough adversary, regularly targeting state institutions, the army and foreigners.

Local Sunni militants have also taken encouragement from the rapid gains in Iraq and Syria made by Islamic State, a group that split from al Qaeda and is now a rival for the hearts and minds of jihadists around the world.

Ansarullah leader Abdel-Malek al-Houthi said the involvement of al Qaeda was evidence of foreign intervention in Yemen.

"Zaydis and Shafi'is throughout history lived side by side as brothers in Islam," Abdel-Malek said in a speech in October.

"What some are trying to stir up today comes essentially in a political context, as the country passes through a sensitive, important and exceptional phase."

Ambassadors from 10 Western and Gulf Arab countries that make up an informal body seeking to nudge forward Yemen's political transition, said in a joint statement they were "increasingly concerned the fighting between Ansarullah and al Qaeda in the Arabian Peninsula risks plunging Yemen into a broader conflict."

The historian, Taher, said much of the blame for the crisis lay with the power transfer deal -- backed by Gulf Arab states and the United Nations -- that eased Saleh from office in 2011.

The deal prevented all-out conflict, but kept in place much of Saleh's administration.

National reconciliation talks organised by Saleh's successor, President Abd-Rabbu Mansour Hadi, were an attempt to find a way out of the crisis, Taher said.

"But there was a failure to properly restructure the armed forces and reform the political and economic conditions," he said. "The result is traditional powers are jostling for their share in the future government and are ignoring the (sectarian) struggle that is tearing civil society apart."

The Houthis portray their capture of Sanaa as an uprising against a central government paralysed by corruption. They point to its failure, three years after Saleh was ousted, to improve living conditions for ordinary Yemenis.

But the fact that the city of 2 million fell with little resistance from government troops has raised speculation that the Houthis received tacit support from Saleh, who originally comes from a Zaydi clan but follows the Sunni faith, and still has loyalists inside the security establishment.

Some diplomats say Saleh struck a tactical alliance with the Houthis to derail the transition and hurt political rivals.

The United States has asked the U.N. Security Council for targeted sanctions against Saleh, accusing the former president of supporting the Houthis and being behind previous attempts to cause chaos across Yemen, a charge he denies.

Sectarianism is not the only rising threat to Yemen. The fall of Sanaa has raised separatist demands in the south, where some leaders aim to resurrect the socialist state that existed before a merger with the northern half of the country in 1990.

There are similar demands in the western coastal region of Tihama, home to the country's second biggest port, Hodeida.-Reuters