IRBIL, Iraq — Iraq appeared closer to its long-predicted disintegration Thursday as Kurdish leaders ordered steps to hold a referendum “as soon as possible” on self-rule for their oil-rich territory and Islamist extremists improved their position by seizing additional territory across the border in Syria.

U.S. military officials, offering their first public assessment of the situation, said it will take decades to subdue the threat now posed by the Islamic State, which seized Iraq’s second-largest city, Mosul, June 10, and then raced southward 200 miles, coming to within an hour’s drive of Baghdad before its advance was halted. On Sunday, the group declared an Islamic caliphate on the land it controls in Syria and Iraq.

Army Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, said in Washington that the Islamic State’s rapid advance had “stretched” its resources. Still, he said he believed the world would have to wrestle with the organization for “a generation or two.” He also labeled as “bleak” the future for a united Iraq unless there is reconciliation between Sunni Arabs, who have supported the Islamic State’s advances, and majority Shiite Arabs, who run the government.

The impact of the Islamic State’s success became clear Friday in Syria, where Islamist fighters took over five towns in the Deir el Zour region without a fight. The development allowed the Islamic State to secure its rear flanks, where it had been battling other Syrian rebel groups, and gain control over much of Syria’s oil and natural-gas production.

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The Islamic State was able to make the advance in Syria after the Nusra Front, al-Qaida’s official Syrian affiliate and a bulwark of the movement to topple the government of President Bashar Assad, abandoned its positions, according to the Syrian Observatory for Human Rights, a Britain-based group that monitors the Syrian war.

It was what took place in Irbil on Thursday that was most likely to cement Iraq’s partition into regions along ethnic and sectarian lines.

Massoud Barzani, president of the Kurdistan Regional Government, buoyed by Israeli Prime Minister Benjamin Netanyahu’s public support for independence, took the first legal steps toward holding a referendum on self-rule, instructing the regional Parliament to pass a bill setting up an election commission “as soon as possible.”

“We have international support for independence, and those who do not support us do not oppose us,” he said in an official statement.

Barzani also said Kurdish military forces would not withdraw from Kirkuk and other disputed areas outside Kurdistan’s borders that it occupied after the collapse of the Iraqi army.

“It is time to decide about our self-determination and not to wait for other people to decide about us,” he said, according to audio of the closed-door remarks obtained by The Associated Press.