US President Barack Obama is set to outline an expanded military and political effort to combat the Islamic State armed group in Syria and Iraq, and urge American legislators to quickly give him authority to arm Syrian opposition forces.

Obama’s much-anticipated television address on Wednesday comes on the eve of the 13th anniversary of the September 11 attacks on the US, which drew America into overseas wars and a still unending “long war”

Officials in his administration said Obama would press forward with other elements of his plan without authorisation from the US Congress.

The Associated Press news agency reported that Obama’s new strategy against the Islamic State could include air strikes in Iraq and possibly in Syria.

Other elements of Obama’s plan include increased support for the Iraqi security forces, as well as military and diplomatic commitments from partners in Europe, the Middle East and elsewhere.

After an hour-long discussion with congressional leaders on Tuesday, the White House said Obama told legislators that he had the authority he needed to take action against the Islamic State.

The speech will represent a chance for Obama to redress criticism that he has been slow to respond to the Islamic State challenge, amid fears fighters with Western passports could hit US targets.

Obama dented his credibility two weeks ago when he admitted he did not yet have a strategy against the Islamic State group in Syria.

But Al Jazeera’s Patty Culhane, reporting from Washington DC, said that Obama has the support of the American public, with 91 percent of them viewing the Islamic State as a threat to the US.

Obama started the work of creating an international coalition to take on the Islamic State at the NATO summit last week.

Meanwhile, three car bombs exploded in Baghdad killing at least 15 people and wounding 29 others on Wednesday, as US Secretary of State John Kerry was visiting the Iraqi capital.

On Thursday, Kerry will head to Saudi Arabia to meet foreign ministers from the Arab states to continue efforts to form a coalition against the Islamic State group. Al Jazeera