BAGHDAD/WASHINGTON (Reuters) - U.S. President Barack Obama authorized air strikes on Iraq to protect Christians and prevent "genocide" of tens of thousands of members of an ancient sect sheltering on a desert mountaintop from Islamic State fighters threatening to exterminate them.
In Baghdad, where politicians have been paralyzed by infighting while the state falls apart, the top Shi'ite cleric all but ordered Prime Minister Nuri al-Maliki to quit, a bold intervention that could bring the veteran ruler down.
The United States began to drop relief supplies to refugees from the ancient Yazidi sect, but there was no sign yet of air strikes, which Obama authorized for the first time since pulling troops out in 2011.
Sunni fighters from the Islamic State, an al Qaeda offshoot bent on establishing a caliphate and eradicating unbelievers, have swept through northern Iraq since June. Their advance has dramatically accelerated in the past week when they routed Kurdish troops near the Kurdish autonomous region in the north.
Hundreds of thousands of Iraqi Christians and other minorities have fled from Islamic State fighters who have beheaded and crucified some of their captives and broadcast the killings on the Internet.
The retreat of the Kurds has brought the Islamists to within a half hour's drive of Arbil, the prosperous capital of the Kurdish autonomous region and a hub for U.S. and European oil companies, which ordered emergency evacuations of their staff.
"Earlier this week, one Iraqi in the area cried to the world, 'There is no one coming to help'," said Obama in a late night TV address to the nation on Thursday. "Well, today America is coming to help."
"We can act carefully and responsibly to prevent a potential act of genocide," he said.
While the relentless advance of Islamic State fighters has threatened to destroy Iraq as a state, bickering politicians in Baghdad have failed to agree on a new government since an inconclusive election in April.
Maliki, a Shi'ite Islamist whose foes accuse him of fuelling the Sunni revolt by running an authoritarian sectarian state, has refused to step aside for a less polarizing figure, defying pressure from Washington and Tehran.
Grand Ayatollah Ali al-Sistani, a reclusive 84-year-old scholar whose word is law for millions of Shi'ites in Iraq and beyond, has repeatedly pushed for politicians to break the deadlock and reunify the country.
His weekly sermon on Friday, read out by an aide, was his clearest call for Maliki to go. Though he did not mention Maliki by name, he said politicians who cling to posts were making a "grave mistake", and leaders must choose a prime minister to end the security crisis.
(Additional reporting from Isabel Coles in Arbil and Michael Georgy in Baghdad and Michael Shields in Vienna; Writing by Paul Taylor and Peter Graff, editing by Peter Millership)