WASHINGTON - US President Barack Obama will lay out his plan for combating the Islamic State of Iraq and Syria (ISIS) during a nationally televised speech from the White House on Wednesday night.

The speech, set for 9 pm local time (6 a.m. PST Thursday), will focus on his much-anticipated strategy for “degrading and ultimately destroying the terrorist group,” the White House said in a statement.

According to media reports, people briefed on the president’s plans described a long-term campaign far more complex than the targeted strikes the United States has used against Al Qaeda in Yemen, Pakistan and elsewhere.

Obama has resisted military engagement in Syria for more than three years, out of fear early on that arming the rebels who oppose President Bashar al-Assad would fail to alter the balance in the civil war while more direct military intervention could have spillover effects in the volatile region.

When he threatened Syria with a missile strike last year after Syrian forces allegedly used chemical weapons, implacable opposition in Congress led him to shelve the plan. Now, however, the threat from ISIS has changed both the American political climate and his calculations.

On Tuesday, the president briefed Republican and Democratic leaders of the House and the Senate about his plans. Obama told them he believed he had the authority needed to order an expanded operation, though he would “welcome action by the Congress that would aid the overall effort,” the White House statement said.

But Congress is divided on the need for a vote on military action before the midterm elections, and both sides appeared to be searching for a way to enlist congressional support without an explicit authorization of force. One way under discussion would be for lawmakers to approve $500 million in funding to train and arm Syrian rebels who would fight ISIS — legislation that has been languishing on Capitol Hill.

Obama’s speech to the nation, on the eve of the 13th anniversary of the Sept 11, 2001, terrorist attacks, is the culmination of weeks of anguished internal deliberations, followed by days of intense lobbying of allies by the president — at a NATO meeting in Wales, with Congress, and even over a three-hour dinner Monday night with members of the Democratic and Republican foreign-policy establishment. Congressional leaders “expressed their support for efforts to degrade and ultimately destroy” the Islamic State, the White House said.

House Speaker John Boehner, a Republican, one of four congressional leaders who met with Obama, expressed support for some of the president’s options, “such as increasing the effectiveness of the Iraqi Security Forces and training and equipping the Syrian opposition,” said an aide to the speaker.

Boehner also said “he would support the President if he chose to deploy the military to help train and play an advisory role for the Iraqi Security Forces and assist with lethal targeting” of the Islamic State leadership, said the aide who spoke on condition of anonymity because it was a private meeting.

The president will use the speech to discuss “the progress that we have made thus far” against insurgents, including ongoing airstrikes in Iraq and formation of a national government in Baghdad, White House spokesman Josh Earnest said.

Obama will focus on the “next phase,” Earnest said, including help to the Iraq military and to moderate forces in Syria so they can “take the fight” to Islamic State insurgents. Obama, he said, will discuss assistance from other countries in the battle against the Islamic State, also known as ISIL and ISIS. Last week in Wales, Obama said a group of nine countries joined to fight the Islamic State.

Obama may raise the long-term potential for airstrikes in Syria, though there is no sign that any such action is imminent. Military surveillance flights have started over Syria, where Earnest said the Islamic State has a “virtual safe haven.”

The administration does not plan to send American troops there or to act without support from allies, Earnest said.

Obama told NBC’s Meet the Press over the weekend that he will not send ground troops into combat, and this strategy will not be the equivalent of the Iraq War launched in 2003.

This plan “is similar to the kinds of counter-terrorism campaigns that we’ve been engaging in consistently over the last five, six, seven years,” Obama said. He said, “The next phase is now to start going on some offence.” The Islamic State has captured large sections of Syria and Iraq. Obama and other Western officials say it plans to use its “caliphate” to launch attacks on US and European interests.