WASHINGTON — The Obama administration is preparing to carry out a campaign against the Islamic State that may take three years to complete — requiring a sustained effort that could last until after President Obama has left office, according to senior administration officials.

The first phase, an air campaign with nearly 145 airstrikes in the past month, is already underway to protect ethnic and religious minorities and US diplomatic, intelligence, and military personnel, and their facilities, as well as to begin rolling back Islamic State gains in northern and western Iraq.

The next phase, which would begin sometime after Iraq forms a more inclusive government, scheduled this week, is expected to involve an intensified effort to train, advise, or equip the Iraqi military, Kurdish fighters, and possibly members of Sunni tribes.

The final, toughest, and most politically controversial phase of the operation — destroying the terrorist army in its sanctuary inside Syria — might not be completed until the next administration. Indeed, some Pentagon planners envision a military campaign lasting at least 36 months.

Obama will use a speech to the nation Wednesday to make his case for launching a US-led offensive against Sunni militants gaining ground in the Middle East, seeking to rally support for a broad military mission while reassuring the public he is not plunging US forces into another Iraq War.

“What I want people to understand,” Obama said in an interview with NBC’s “Meet the Press” that was broadcast Sunday, “is that over the course of months, we are going to be able to not just blunt the momentum” of the militants. “We are going to systematically degrade their capabilities; we’re going to shrink the territory that they control; and, ultimately, we’re going to defeat them,” he added.

The military campaign Obama is preparing has no obvious precedent. Unlike US counterterrorism operations in Yemen and Pakistan, it is not expected to be limited to drone strikes against militant leaders. Unlike the war in Afghanistan, it will not include the use of ground troops, which Obama has ruled out.

Unlike the Kosovo war that President Clinton and NATO nations waged in 1999, it will not be compressed into an intensive 78-day tactical and strategic air campaign. And unlike the air campaign that toppled the Libyan leader, Moammar Khadafy, in 2011, the Obama administration is no longer “leading from behind,” but plans to play the central role in building a coalition to counter the Islamic State.

“We have the ability to destroy ISIL,” Secretary of State John Kerry said last week at the NATO summit meeting in Wales, using an alternative name for the militant group. “It may take a year, it may take two years, it may take three years. But we’re determined it has to happen.”

Antony Blinken, Obama’s deputy national security adviser, has suggested that the United States is undertaking a prolonged mission. “It’s going to take time, and it will probably go beyond even this administration to get to the point of defeat,” Blinken said last week on CNN.

Kerry is scheduled to head for the Middle East soon to solidify the anti-Islamic State coalition. And Defense Secretary Chuck Hagel is traveling to Ankara, Turkey, on Monday to woo another potential ally in the fight against the Sunni militant group.

Although details of how the emerging coalition would counter the Islamic State remain undecided, several US officials said that they believe the list of allies so far includes Jordan, offering intelligence help, and Saudi Arabia, which has influence with Sunni tribes in Iraq and Syria and which has been funding moderate Syrian rebels.

The United Arab Emirates, officials said, has also indicated a willingness to consider airstrikes in Iraq. Germany has said it would send arms to peshmerga fighters in Kurdistan. And rising concern over foreign fighters returning home from Syria and Iraq may also have spurred Britain, Australia, France and Denmark to join the alliance.

Administration officials acknowledged, however, that getting those same countries to agree to airstrikes in Syria was proving harder.

“Everybody is on board Iraq,” an administration official said, speaking on condition of anonymity because the policy is still being developed.

“But when it comes to Syria, there’s more concern” about where airstrikes could lead, the official said.

The official nonetheless expressed confidence that the countries would eventually come around to taking the fight into Syria, in part, he said, because “there’s really no other alternative.”

The United States launched a new series of airstrikes against Sunni fighters in Iraq late Saturday in what Defense Department officials described as a mission to stop the militants from seizing the Haditha Dam on the Euphrates River and prevent the possibility of floodwaters being unleashed toward the capital, Baghdad.

The attacks were aimed at militant fighters of the Islamic State as they were moving toward the dam, officials said. The operation represented another expansion of the limited goals that Obama set out when he announced last month that he had authorized airstrikes in Iraq.

Administration officials nonetheless stressed that the strikes around the dam — about 175 miles from Baghdad, in Anbar province — were within the constraints of what Obama initially characterized as a limited campaign against the militants.

The goal of the initial US campaign was to break the Islamic State siege of the minority Yazidi population stranded on Mount Sinjar, as well as to protect US citizens, official personnel, and facilities in Erbil, the Kurdish capital, and Baghdad.

The US warplanes struck what were described as Islamic State positions in the towns of Rawa and Ana, as well as in Barwana, which is about 9 miles from the dam. The strikes on Rawa and Ana, towns near a highway leading into Syria, could disrupt Islamic State supply lines in Iraq.

Soldiers fighting there said the US planes struck again on Sunday, hitting what they described as houses where militant fighters were gathering.

Obama’s planned speech suggests that he may be moving closer to a decision on many remaining questions, including whether and at what point the White House might widen the air campaign to include targets across the border in Syria, possibly to include Islamic State leadership, and its equipment, supply depots and command centers.

The time of the speech Wednesday has not been announced.

Senior officials have repeatedly ruled out sending ground combat troops, a vow Obama reaffirmed in his appearance on “Meet the Press.”