WASHINGTON – In a political challenge to a country already under assault by an extremist Sunni army, Iraq’s longtime prime minister, Nouri al-Maliki, is refusing to cede control as his term of office nears its end, suggesting over the weekend a willingness to use force to stay in power.

The power struggle in Baghdad is compounding a military crisis in northern Iraq, where the Islamic State, a radical religious militia holding swaths of Iraqi and Syrian territory, has challenged the control of the government.

At the invitation of that government and citing a moral imperative, the United States continued a military assault on Islamic State assets on Monday, including targets outside the city of Erbil as well as its first airstrikes against targets near Mount Sinjar. One series of bombings, at the base of the mountain refuge for religious minorities, included checkpoints, trucks, and US-made humvees commandeered and operated by the terrorist network.

Earlier on Monday, Iraq’s President Fouad Masoum nominated Haider al-Abadi as Maliki’s successor, after strong encouragement from Washington to abide by the country’s decade-old constitution.

Speaking to reporters from Martha's Vineyard, US President Barack Obama congratulated Masoum and Abadi, encouraging the leadership to "unite Iraq's different communities" and to "form a new cabinet as quickly as possible."

Reiterating America's commitment to the Iraqi people, Obama repeated his belief that a diverse, representative government in Baghdad was a necessary partner to meet the difficult task of confronting the Islamic State.

Abadi is a Shi’ite and a member of Maliki’s Dawa Party.

But Abadi’s own party colleagues publicly rejected his appointment on Monday, charging that his nomination had “no legitimacy” and that Abadi “only represents himself.”

In a demonstration of his anger, Maliki – still technically prime minister after eight years in power – ordered a show of force on the streets of Baghdad on Sunday leading up to the announcement, which was welcomed by the US, United Kingdom and United Nations.

The US has noticed “no discernible change” in the security presence in Baghdad, State Department deputy spokeswoman Marie Harf told reporters in Washington, adding that Maliki is still the country’s prime minister.

Abadi has 30 days to form and present a new government before Maliki officially steps down.

US Vice President Joe Biden called Masoum and Abadi on behalf of the United States, to congratulate them on the step forward.

“The prime minister-designate expressed his intent to move expeditiously to form a broad-based, inclusive government capable of countering the threat of the Islamic State of Iraq and the Levant, and building a better future for Iraqis from all communities,” the White House said.

US Secretary of State John Kerry came out with a statement congratulating Abadi — and warning Maliki not to “stir the waters” with violence in the streets of the Iraqi capital.

UN secretary-general Ban Ki-moon extended his congratulations, but with a rare critique of the internal politics of a member state.

Ban “is concerned that heightened political tensions coupled with the current security threat of Islamic State could lead the country into even deeper crisis,” his spokesman said.

The US, meanwhile, continued its aid airdrop on Mount Sinjar, where thousands of members of the Yazidi religious minority remain trapped by IS fighters bent on their extermination.

A fourth US airdrop of ready-to-eat meals, tents and thousands of gallons of water successfully landed on the mountaintop overnight, as London committed to the effort with fighter jets to guide its own cargo planes full of aid.

The greater question of how to secure safe passage for the Yazidis is still challenging the US military, however, which is “right now gripped by the immediacy of the crisis,” in the words of one Pentagon official.

“We’re currently assessing what we can and can’t do,” a Pentagon spokesman said.

“What is most important right now is that we deliver the much-needed water and shelter and food.”

But the White House reiterated its commitment to the prevention of genocide against the Yazidis, which was one of two primary justifications Obama cited last week as he authorized the use of force against the Islamic State.

“We’re reviewing options for removing the remaining civilians off the mountain,” deputy US national security adviser Ben Rhodes said. “Kurdish forces are helping, and we’re talking to the [United Nations] and other international partners about how to bring them to a safe space.”

The UN mission in Iraq said it is preparing a humanitarian corridor to permit the Yazidis to flee to safety.

Yazidis are followers of an ancient religion derived from Zoroastrianism. They are viewed as “devil worshipers” by the Islamic State’s Sunni extremists, who ordered them to convert to Islam or face death.

US fighter jets continued firing on the Islamic State on Sunday, striking several vehicles en route to Erbil, where the US maintains a consulate.

The State Department said the US has begun to arm Kurdish fighters in “full cooperation and coordination” with Baghdad, to help defend the northern territories with ground forces.

Reuters contributed to this report.

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