By Mohammed Ghobari and Mohammed Mukhashaf
SANAA/ADEN (Reuters) - Islamic State claimed responsibility for suicide bombings in the Yemeni capital Sanaa that killed 126 people during Friday prayers at two mosques used by supporters of Shi'ite Houthi fighters who now control the capital.
The attacks were the deadliest in a years-long campaign of militant violence in the country, where Washington has been waging a drone air war against a powerful local branch of the Sunni Muslim militant group al Qaeda.
Sectarian unrest has increased in recent months after Iran-backed Shi'ite fighters seized the capital last year.
Four bombers wearing explosive belts targeted worshippers in and around the crowded mosques. Hospitals were overwhelmed by the dead and wounded, appealing for blood donors to help treat the large number of casualties.
A Reuters journalist at the Badr mosque counted at least 25 bloody bodies lying in the street and inside the building. One man carried a child in his arms. Medical sources gave the death toll of 126 and said at least 250 other people were wounded.
Al Qaeda and Islamic State, the al Qaeda offshoot that now controls swathes of Syria and Iraq, consider all Shi'ites to be heretics. They have now rallied against the Houthis in Yemen, giving them the same enemies as the U.S.-backed government in a complex, multi-sided conflict in the Arab world's poorest country.
"Let the polytheist Houthis know that the soldiers of the Islamic State will not rest and will not stay still until they extirpate them," the group said in a statement posted by supporters on Twitter, claiming responsibility for the attacks.
"God willing, this operation is only a part of the coming flood."
Television pictures showed young men in traditional Yemeni clothes carrying lifeless bodies, some dripping with blood, out of the mosque.
One witness said he heard two successive blasts at the Badr mosque, in a busy neighbourhood in central Sanaa.
"I was going to pray at the mosque then I heard the first explosion, and a second later I heard another one," the witness told Reuters.
Two bombers hit Badr and two others hit second mosque. Another bomber also tried to target a mosque in the northern Houthi stronghold of Sadaa province, but the bomb went off prematurely, killing only the bomber, a security source told Reuters.
In Washington, the White House condemned the bombings and said it could not confirm that the attackers were affiliated with Islamic State.
HURTLING TOWARDS CIVIL WAR
Yemen has been hurtling towards civil war since last year, when the Iran-backed Shi'ite fighters known as Houthis seized most of the north, including the capital Sanaa.
President Abd-Rabbu Mansour Hadi, a U.S. ally, fled the capital in February after a month imprisoned by the Houthis under house arrest and has set up a power base in the southern city of Aden.
Unidentified warplanes have attacked his Aden palace for the past two days.
Anti-aircraft guns fired on two planes which dropped bombs on an area that includes his residence on Friday. He was unharmed, sources at the presidency said.
While Yemen is one of the main bases of al Qaeda, it has not previously been known as a major base for Islamic State, the Al Qaeda offshoot also known as ISIS or ISIL.
Since last year, when Islamic State swept across northern Iraq and declared a caliphate to rule over all Muslims, militants in other countries have expressed their support for the group, although it is not clear if it actually directs them.
In Washington, White House spokesman Josh Earnest said there was no clear operational link between the people who carried out Friday's attacks in Yemen and Islamic State fighters in Iraq and Syria.
The United Nations Secretary-General Ban Ki-moon condemned "the terrorist attacks" in Yemen and called on all sides "to immediately cease all hostile actions and exercise maximum restraint."
Yemen has been sliding into turmoil since its long serving ruler Ali Abdullah Saleh was toppled after "Arab Spring" protests that began in 2011. Saleh is now believed to have allied himself with the Houthi fighters that he tried to crush while president.
Since fleeing the capital, Hadi has been trying to consolidate his hold over Aden to challenge the Houthis' ambitions to control the whole country.
Thirteen people were killed on Thursday when forces loyal to Hadi fought their way into Aden's international airport and wrested an adjacent military base from a renegade officer, Aden governor Abdulaziz bin Habtoor said.
(Additional reporting by Sami Aboudi in Dubai and Omar Fahmy in Cairo; Writing by Rania El Gamal; Editing by William Maclean and Peter Graff)
