US President Barack Obama will 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 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 will press forward with other elements of his plan without authorisation from the US Congress.

The Associated Press news agency reported that could include air strikes in Iraq and possibly in Syria.

Other elements of Obama’s plan include increased support for 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 Tuesday, the White House said Obama told legislators that he has the authority he needs 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 armed with Western passports could hit US targets.

Obama dented his credibility two weeks ago when he admitted he did not yet have a strategy to take on 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.

Chuck Hagel, US defence secretary, has been in the Middle East, and John Kerry, US secretary of state, arrived in Baghdad on Wednesday to accelerate the administration’s efforts.

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.

Mindful of avoiding what he believes are the mistakes of the last decade, White House officials said Obama will assure millions of television viewers that he will not send conventional ground troops back to Iraq to fight a group that has beheaded two US journalists.

The speech will also lack a definitive timeline for US operations against Islamic State fighters, after several reports cited senior officials as saying they could outlast Obama’s presidency, which ends in January 2017.

“I think the American people need to expect that this is something that will require a sustained commitment,” Josh Earnest, White House spokesman, said.

Obama, who sees ending wars in Afghanistan and Iraq as a centerpiece of his legacy, is also under pressure to announce stepped-up support for moderate rebels in Syria, despite his reluctance to intervening in Syria’s civil war.

The US wants to ensure that President Bashar al-Assad, who it regards as a war criminal, does not benefit from any power vacuum left in the event that US military action degrades the Islamic State.