BAGHDAD — In the week since the last American troops left Iraq, Shiite Prime Minister Nouri al-Maliki ordered an arrest warrant for the country’s highest-ranking Sunni official, threatened to exclude the rival sect’s main political party from his government and warned that “rivers of blood” would flow if Sunnis seek an autonomous region.

The moves confirmed what many longtime observers of Iraqi politics have suspected since al-Maliki came to office more than five years ago — that he has an authoritarian streak and beneath his tireless rhetoric about national unity is essentially a sectarian politician.

As a result, the veneer of sectarian unity that the United States tried to paint over Iraq’s leadership throughout a 9-year presence is quickly being washed away after the departure of American forces.

The first casualty could be the unity government that al-Maliki heads, uneasily combining his powerful Shiite alliance with a Sunni-backed bloc. It took nine months after Iraq’s elections in March last year to put it together, under heavy American pressure to include the Sunnis, but al-Maliki never liked it and is increasingly saying he wants a government based on the majority in parliament, which would squeeze out Sunnis.

And al-Maliki has made clear he intends keep a strong grip heading that government.

“I have been working here for six years and I will be here for another six,” al-Maliki told a news conference last week.

He has been accruing power since rising to his post in 2006 in a process that has accelerated since the new government was formed a year ago. He effectively runs the Defense and Interior Ministries and has created a separate security force that answers to him alone. He has bypassed parliament to install Shiite allies in key positions, and he has used his control over state funds and resources to gain leverage with the judiciary and oversight agencies like the anti-graft Integrity Commission.

The one risk al-Maliki runs is that he will disillusion Shiite parties making up the bulk of his government, some of which are longtime political rivals. On Monday, a lawmaker in the powerful party of radical Shiite cleric Muqtada al-Sadr floated the idea of holding new elections to resolve the political turmoil. Bahaa al-Aaraji quickly backed off the idea, saying it was just his personal opinion, but his comment underlined how Shiite rivals could turn on the prime minister.

Nevertheless, for the moment, Shiite parties are strongly backing al-Maliki, unified by their common fear that Sunnis want to take back power that their minority community lost with the 2003 fall of Saddam Hussein’s Sunni-dominated regime.

Last week, the Shiite bloc issued a statement saying rulings of the judiciary must be adhered to, language that was seen as a show of support for al-Maliki’s arrest warrant against Sunni Vice President and longtime critic Tariq al-Hashemi.

Al-Maliki accuses al-Hashemi of organizing assassinations, and the warrant has sparked a political crisis. Al-Hashemi denied the charges. The Iraqiya bloc to which he belongs is boycotting parliament, complaining that al-Maliki does not share power, and it is threatening to pull out of the unity government.

The prime minister also asked parliament to issue a vote of no-confidence in Iraqiya lawmaker Saleh al-Mutlaq in what could be a prelude to removing or prosecuting him. Al-Mutlaq, a deputy prime minister, branded al-Maliki a dictator in a TV interview while the Shiite leader was visiting Washington this month.

For Sunnis, the moves stoke fears that the majority Shiites want to exclude them from politics and completely dominate the country. They also worry that increasing Shiite power will translate into greater influence from neighboring, Shiite-led Iran. On Sunday, Iran’s armed forces chief of staff Gen. Hassan Firouzabadi said his country was ready to expand its military, defense and security relations with Iraq now that the Americans have left, according to Iran’s state news agency IRNA.

Al-Maliki, meanwhile, has been able to exploit Shiite fears of a Sunni resurgence to keep his coalition’s backing while he expands his personal role. Helping him is the lack of any credible Shiite rival, with former prime minister Ibrahim al-Jaafari and former vice president Adil Abdel-Mahdi viewed as too weak to take his job.