By Roberta Rampton

WASHINGTON, Sept 7 (Reuters) - President Barack Obama said he will explain to Americans and congressional leaders this week his plan to "start going on some offense" against Islamic State militants, who he said could eventually become a threat to the United States.

Obama will make a speech on Wednesday to "describe what our game plan's going to be," and meet congressional leaders on Tuesday to seek their support for his strategy to halt the militant Islamist group, which controls parts of Syria and Iraq.

The president, who campaigned on getting U.S. troops out of Iraq, has struggled to articulate how he wants to address Islamic State, telling reporters last month that "we don't have a strategy yet" to tackle the group.

"I just want the American people to understand the nature of the threat and how we're going to deal with it and to have confidence that we'll be able to deal with it," Obama said in an interview with NBC's "Meet the Press" that aired on Sunday. The interview was conducted in Washington on Saturday.

"The next phase is now to start going on some offense," he said.

The Wednesday speech will come a day ahead of the anniversary of the Sept. 11, 2001, attacks, when al Qaeda militants flew hijacked planes into the World Trade Center in New York and the Pentagon, killing almost 3,000 people.

"I want everybody to understand that we have not seen any immediate intelligence about threats to the homeland" from the Islamic State group, Obama said.

But the group has attracted foreign fighters from Western nations who could travel to the United States "unimpeded," Obama said. "Over time, that can be a serious threat to the homeland," he said.

In an interview earlier this year, Obama had put the group in a category of foreign militant movements that were a minor threat, comparing it to a "JV", or junior varsity, team. But he told NBC the group had grown. "They're not a JV team," he said.

OBAMA'S STRATEGY

He ruled out sending U.S. ground troops to fight in Iraq or Syria, saying "this is not the equivalent of the Iraq war," and described the coalition he spent time building last week at a NATO meeting in Wales.

This is going to be "similar to the kinds of counterterrorism campaigns that we've been engaging in consistently over the last five, six, seven years," Obama said.

"We are going to be a part of an international coalition, carrying out air strikes in support of work on the ground by Iraqi troops, Kurdish troops," Obama said. Nine other countries have agreed to be "core" members of the coalition.

Secretary of State John Kerry and Defense Secretary Chuck Hagel are engaged in a mission in the region to flesh out the plan. "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 'em," Obama said.

In the interview, Obama did not say whether he would authorize air strikes in Syria. "The strategy both for Iraq and for Syria is that we will hunt down ISIL members and assets wherever they are," he said, using an acronym for the Islamic State.

He emphasized the United States would need Sunni Arab states in the region including Saudi Arabia, Jordan, the United Arab Emirates and Turkey to "step up" and help.

"I think, for ... perhaps the first time, you have absolute clarity that the problem for Sunni states in the region, many of whom are our allies, is not simply Iran. It's not simply a Sunni-Shia issue," he said. Iran is a Shiite-dominated country.

Obama wants regional allies to help win over and work with disaffected Sunni tribes in Iraq - an effort he said could include an "economic element."

Obama has faced criticism for appearing disengaged on the Iraq crisis, particularly after he played a round of golf minutes after speaking about the beheading of American journalist James Foley by Islamic State militants.

Obama told NBC he wished he could get "a vacation from the press" but admitted the golf game was a bad decision.

"I should've anticipated the optics," he said, explaining the "theater" involved with being president is "not something that always come naturally to me." (Reporting by Roberta Rampton; Editing by David Story and Will Dunham)