WASHINGTON — In an address to the nation, President Barack Obama will outline an expanded military and political effort to combat militants in Syria and Iraq, and urge Congress to quickly give him authority to arm moderate Syrian opposition forces fighting President Bashar Assad.

But administration officials said Obama will press forward with other elements of his plan without formal authorization from lawmakers. That could include wide-ranging airstrikes in Iraq and possibly in Syria. Other elements of Obama’s plan, which he was to lay out in a prime-time TV speech today, included increased support for Iraqi security forces, as well as military and diplomatic commitments from partners in Europe, the Middle East and elsewhere.

After an hourlong discussion with congressional leaders Tuesday, the White House said Obama told lawmakers that he “has the authority he needs to take action” against the Islamic State militants. The White House added that the president still would welcome action from Congress that would “aid the overall effort and demonstrate to the world that the United States is united in defeating the threat.”

For Obama, a sustained U.S. intervention in the Middle East is at odds with the vision he had for the region when he ran for president on a pledge to end the war in Iraq, where the role of American fighting forces drew to a close nearly three years ago. The timing of his announcement tonight was all the more striking, scheduled just hours before anniversary commemorations of the terrorist attacks of Sept. 11, 2001.

Among the president’s most urgent priorities will be seeking authorization from Congress to arm more moderate elements of the Syrian opposition fighting Assad. The president asked lawmakers earlier this year for a $500 million train-and-equip program, but the plan stalled on Capitol Hill.