WASHINGTON — President Barack Obama will outline in an address to the nation tonight an expanded military and political effort to combat Islamic State militants in Syria and Iraq, possibly including airstrikes in Syria, officials said.

The president will also 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 priorities without formal authorization from lawmakers. That could include wide-ranging airstrikes in Iraq and perhaps Syria as well. Other elements of Obama’s plan 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.”

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.

The U.S. already has been running a smaller CIA program to train the rebels, but Obama is seeking approval for a more overt military effort.

With Obama ruling out sending U.S. ground troops into combat in Iraq or Syria, bolstering the capacity of the Iraqi security forces and Syrian opposition will be crucial to efforts to root out the Islamic State militant group, which has moved freely across the blurred border between the two countries. U.S. airstrikes could help give the forces in both countries the space to make gains against the extremists.

Administration officials said Obama also sees a congressional authorization for a Syrian train-and-equip message as sending a strong signal to allies who are considering similar efforts.