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Suicide bombers affiliated with the Islamic State have killed at least 126 people and injured many more during Friday prayers at two Shia mosques in Yemen's capital city Sanaa.
The Badr and al Hashoosh mosques were hit while they were packed with supporters of the Houthis, a Shia movement from Yemen's north which took control of Sanaa in January. The attacks immediately prompted fears of the emergence of a kind of sectarian violence which has largely been absent from Yemen.
A statement in the name of a group styling itself the Sanaa State, the local section of Islamic State, claimed responsibility for the attacks.
The prominent Houthi imam, Murtada Al-Mohatwari, was among those killed, according to the state news agency SABA.
WARNING: THIS VIDEO CONTAINS EXTREMELY GRAPHIC IMAGES
The Houthis are opposed by several forces with their roots in the largely Sunni south, including al Qaeda in the Arabian Peninsular and a growing band of Islamic State supporters.
An AQAP spokesperson denied they had carried out the attacks, telling VICE News: "It is forbidden in AQAP to target any mosque, even if Obama is in there."
Islamic State currently differs with their al Qaeda counterparts in Syria by their more ready embrace of brutal sectarian tactics, such as suicide bombings in the vicinity of mosques used by Shiites.
Al Qaeda, whose Iraqi affiliate pioneered the technique in the wake of the US invasion, came to officially reject it as alienating public support.
Adam Baron, a visiting fellow at the European Council on Foreign Relations and Yemen expert, said that accounts associated with supporters of IS were celebrating the attack.
Al Jazeera Arabic reported that a separate powerful explosion shook a complex in Saada, a historic Houthi stronghold in the north of the country.
"There has been a risk of a sectarian backlash for sometime but so far Yemen has avoided it," said Baron. "I think there is a widespread fear that this signals Yemen entering the next stage, a stage of sectarian tit-for-tat killing. But it's too soon to tell if that will happen."
"If you look at Yemen's recent history, the country has consistently pulled back from the brink when it looked like it was going off the rails, so there is a possibility that this will end up being an anomaly. Everyone hopes it will be an anomaly. But that looks optimistic. When you look at what this would appear to signal, that would be that Yemen's future is about to get a lot more bloody."
Islamic State in Yemen
Twitter accounts linked to IS supporters initiated the hashtag: "Islamic State strikes Sanaa and Saada."
One said, "the Islamic State begun work... and this was the beginning of that which is blessed."
The bombings follow a drip-feed of hints that IS is a small but growing force in Yemen.
In September, a group calling itself the Supporters of the Islamic State in Yemen released a video of nine fighters swearing allegiance to Islamic State leader Abu Bakr al-Baghdadi.
On November 10 last year, jihadists in Yemen — as well as in Libya, Egypt and Algeria — also pledged their loyalty to Baghdadi, who formally accepted their pledges in an audio address three days later.
The message declared a wilaya — or province — of IS in each country, and declared competing groups, including AQAP, invalid. But in Yemen IS has until now been regarded as marginal.
A Yemeni official told CNN in January that there were "dozens" rather than "hundreds" of IS associates in the country.
Another statement in February appeared to come from AQAP members in central Yemen announcing their defection to IS, according to the SITE Intelligence Group.
"We announce the formation of armed brigades specialized in pounding the apostates in Sanaa and Dhamar," the purported former AQAP supporters wrote.
Sunni jihadists refer to the Shia as apostates.
At least one senior AQAP commander and a tribal leader allied with the group have expressed sympathy with IS, but so far al Qaeda's strongest affiliate has remained independent and avoided major defections.
Witnesses told AFP that at the Badr mosque, in a tactical motif typical of similar attacks in Iraq, one bomber detonated his explosive belt inside the building, while another blew himself up among worshippers fleeing from the first blast, at the mosque gate.
A single bomber struck at the al-Hashoosh mosque.
"This threatens to bring the situation to a whole different level," Baron of the ECFR said. "We are talking about very blunt, unvarnished sectarian killing. This is something that would have been completely unimaginable, even a year ago."
Yemen appears increasingly unstable.
The Houthis, adherents of the Zaidi sect of Shia Islam, stormed into Sanaa in September, prompting the resignation of President Abd Rabbu Mansour Hadi in late January.
Hadi fled south, to the port town of Aden, where he is now locked in a battle with his predecessor Ali Abdullah Saleh.
Saleh was himself ousted from the Presidential Palace in Sanaa during 2011, in the hopeful early days of the Arab Spring, and is now backed by the renegade special forces general Abdel Hafez al-Saqqaf.
On Thursday military aircraft targeted a palace in Aden used by Hadi, who is still described as the President by his supporters. Hadi was evacuated.
Follow Tom Dale on Twitter: @tom_d_