Officials working to shore up support for expanded strategy against Isis in lead up to president’s foreign policy address

The White House said on Tuesday that Barack Obama does not believe he needs congressional authorisation for the “next, more offensive” phase in an ongoing military campaign against jihadist militants in Iraq and Syria.

The administration has been indicating for the past few days that it does not believe that Congress needs to approve any widening of the scope of the campaign against Islamic State (Isis) militants .

There is intense speculation in Washington over whether Obama will expand air strikes to include targets in Syria, where the US president, just a year ago, judged it unwise to intervene militarily.

The White House has scheduled Obama’s address from the State Floor at the White House at 9pm ET, ensuring a rare, primetime television audience for what has been billed as one of the most crucial foreign policy speeches of his presidency.

Republican and Democratic congressional leaders were provided an advance preview of Obama’s speech during an hour-long meeting at the White House on Tuesday.

The Obama administration has ruled out sending ground combat troops into the region, although it has deployed more than 1,200 special operations forces and “advisers” to the region, ostensibly to protect US interests in Baghdad and Irbil.

US officials are working urgently to shore up support for an expanded strategy against Isis at home and abroad – even if, as seems likely, there is no congressional vote over the use of force, the administration wants to secure the political backing of both parties.

In a statement released after the meeting, the Republican speaker of the House, John Boehner, said Obama began “by laying out some of the ideas he has already discussed publicly about how to combat this threat”. However he said he “expressed support for certain options that have been proposed by the president, such as increasing the effectiveness of the Iraqi security forces and training and equipping the Syrian opposition”.

The White House described the meeting as “productive”, saying that while Obama told the congressional leaders he had the legal authority to take action against Isis with the backing from the legislature he would, nonetheless, “welcome action by the Congress that would aid the overall effort and demonstrate to the world that the United States is united in defeating the threat”.

Congress broadly supports expanding the military assault against Isis, with some hawks – particularly on the Republican side – saying the president should consider redeploying ground troops to Iraq. Former vice-president Dick Cheney met behind closed doors with Republican members of the House of Representatives on Tuesday, urging them to adopt a more muscular military posture in the Middle East.

Obama’s televised address is taking place on the eve of the 13th anniversary of the 9/11 attacks – and exactly one year after he delivered another national address, rowing back from what his administration had suggested was imminent military action against Syria over Bashar al-Assad’s use of chemical weapons.

Obama kicked back at the criticism that he had proved a weak commander-in-chief, saying he was ruling out a “prolonged air campaign like Libya or Kosovo” and adding: “I’ve spent four-and-a-half years working to end wars, not to start them.”

The air strikes conducted in Iraq in recent weeks, numbering more than 150, are not yet on the scale of the Libya or Kosovo bombing campaigns, but White House officials have reportedly said the campaign could be expanded in scope and last up to three years.

Some of the president’s advisers are urging him to move to shake his reputation as a weak commander-in-chief who has struggled to contend with a spate of unpredictable foreign conflicts, from Ukraine to Gaza. Democrats fear his failure to show leadership on international affairs could damage the party in November’s midterm elections.

The administration also senses a growing appetite among Americans for a more aggressive military posture in the Middle East.

Two polls released on Tuesday confirmed the shift in public mood. A Washington Post /ABC News survey found that 65% of Americans support airstrikes against jihadists in Syria, more than double the level of backing for strikes against the Syrian regime this time last year. Meanwhile, a Wall Street Journal /NBC News poll found 27% of registered voters would like to see the US play a more active role in world affairs – up 19% since April.

Obama is expected to stress that while the US will lead the drive to build an international coalition to meet his objective of “degrading and destroying” Isis, trusted regional allies will need to lead the on-the-ground battles against the militants.

In Iraq, US officials are determined that military assaults against Isis should be led by Kurdish forces, Sunni tribal fighters and the country’s army. The White House pointed to the formation this week of a new, inclusive government in Baghdad as a turning point for the country.

“Now that they have formed an inclusive government and united the country, there’s an important role for the United States and the United States military to play in supporting the Iraqis as they take the fight to [Isis] in their country,” Obama’s press secretary, Josh Earnest, said on Tuesday.

The US believes the airstrikes conducted against Isis targets across Iraq have been effective at halting the jihadist group’s advance toward Irbil, sending the siege of Mount Sinjar, where thousands of ethnic Yazidis were threatened by militants and, more recently, working in concert with Sunni tribal groups protecting the Haditha dam.

The situation is more complicated in Syria, where there is anxiety in the administration over the possible unintended consequences of intervening in the three-year civil war. US officials have made clear they want to boost support for Syrian opposition groups, but remained vague about any direct military coordination.

Even as the Obama administration fine-tuned its strategy late on Tuesday, doubts are understood to have remained among administration officials, including in the Pentagon, about the wisdom of any military intervention in war-ravaged Syria.

Earnest said would not be drawn on US military activity in Syria, but said leveraging support for Syrian opposition groups was a method “both of taking the fight to [Isis] and also taking the fight to the Assad regime”.