BAGHDAD — Iraq’s incumbent Prime Minister Nouri al-Maliki appeared more isolated today as he pressed his battle to remain in power while Iraqi politicians and the international community rallied behind a Shiite premier-designate who could be a more unifying figure, badly needed if the nation is to confront a spreading Sunni insurgency.

U.S. Secretary of State John Kerry today urged the prime minister-designate, Haider al-Abadi, to work quickly to form an inclusive government and said the U.S. is prepared to offer it significant additional aid in the fight against Islamic State militants.

The power struggle in Baghdad comes as Iraq is battling militants from the al-Qaida breakaway group in the north and the west. The onslaught by the Islamic State, which has captured large chunks of Iraqi territory since June, has become the country’s worst crisis since the U.S. troop withdrawal in 2011.

On Monday, al-Abadi, the deputy speaker of parliament from al-Maliki’s Shiite Dawa party, was selected by President Fouad Massoum to be the new prime minister and was given 30 days to present a new government to lawmakers for approval.

President Barack Obama called al-Abadi’s nomination a “promising step forward” and urged “all Iraqi political leaders to work peacefully through the political process.”

But al-Maliki, who has been in power for eight years, defiantly rejected the nomination, insisting it “runs against the constitutional procedures” and accusing the United States of siding with political forces “who have violated the constitution.”

In Sydney, Kerry said today that the United States “stands ready to fully support a new and inclusive Iraqi government.”

“Without any question, we are prepared to consider additional political, economic and security options,” Kerry said.

The U. S. has already increased its role in fighting back the Sunni extremists who have threatened the autonomous Kurdish region in northern Iraq. American airstrikes have helped the Kurds achieve one of their first victories over the weekend after weeks of retreating. And senior American officials said Monday that U.S. intelligence agencies are directly arming the Kurds who are battling the militants.