Barack Obama will pledge on Wednesday night to "degrade and ultimately destroy" the Islamic State insurgency operating in both Syria and Iraq in an address to the American people expected to herald a significant escalation of the US military role across the region.

Though the exact extent of the anticipated US intervention in Syria remained unclear in the hours leading up to the key speech, White House officials made clear the president planned to pursue a two-pronged strategy on both sides of the border that is likely to build on existing air strikes in Iraq against the group known as Isis or Isil.

"Tonight you will hear from the president how the United States will pursue a comprehensive strategy to degrade and ultimately destroy Isil, including US military action and support for the forces combating Isil on the ground – both the opposition in Syria and a new, inclusive Iraqi government," said a senior administration official in a statement issued to reporters on Wednesday morning.

"The president will discuss how we are building a coalition of allies and partners in the region and in the broader international community to support our efforts, and will talk about how we work with the Congress as a partner in these efforts."

In contrast, Obama's failed attempt to seek congressional backing for US air strikes against Syrian government forces last year, the president is seeking to shore up support on Capitol Hill with further briefings for senators planned on Wednesday but has so far refused to give Congress a veto over his latest strategy.

Ahead of Obama's nationwide address – on the eve of the 13th anniversary of the 9/11 attacks – John Kerry, the US secretary of state, was holding urgent talks in Baghdad with the new Iraqi prime minister. Kerry was expected to press Haider al-Abadi, who was sworn in two days ago, to cede more power to the Sunni majority to bring them on side so Iraq can more effectively confront militants who control a swath of territory straddling Iraq and Syria.

The US is keen that al-Abadi, a Shia, follows through on pledges to give Sunnis more regional authority and greater control over security forces.

Jen Psaki, the State Department spokeswoman, said Kerry and Iraqi government officials would "discuss how the United States can increase its support to Iraq's new government in our common effort to defeat Isil [Isis] and the threat that it poses to Iraq, the region, and the world".

US officials said al-Abadi had promised to create a national guard of local fighters to secure Iraq's 18 provinces – each run by a governor. That would ensure that the Iraqi army and its mostly Shia force would not be in charge of security in Sunni regions. That would bring salaried jobs, government pensions and other benefits to areas of Iraq neglected during al-Maliki's eight years in power and which proved fertile breeding ground for Isis.

After Iraq, Kerry will travel to Saudi Arabia, which has funded groups opposing the Syrian government. The Saudis, while supportive of the US, are fearful that aggressive action against Isis could trigger a backlash among its own Sunni extremists.

The US is working on a strategy it hopes will be ready by the start of the annual meeting of the UN general assembly in New York at the end of the month. Obama was expected to discuss US commitments in his address to the nation.

For weeks, the administration, and particularly the Pentagon, has urged Congress to approve $500m (£310.7m) in funding to train and arm Syrian rebels to fight Isis. The legislation, stuck on Capitol Hill since Obama first sought it in May, also provides both Obama and Congress with an opportunity to tacitly enlist congressional support for a war without a formal vote ahead of the November midterm elections, something many politicians this week indicated they wish to avoid.

The president's broader approach could include more wide-ranging air strikes against targets in Iraq and possibly in Syria, and hinges on military and political commitments from allies in Europe, the Middle East and elsewhere. The UK has sent heavy machine guns and ammunition worth £1.6m to the Iraqi government. The US and nine other countries, including Britain, Canada and Australia, last week agreed to create a united front against Isis.

The US has already launched about 150 air strikes on Isis in Iraq over the past month at the invitation of the Iraqi government and without formal authorisation from Congress. It has also sent more than 1,200 special operations forces and "advisers" to the region, ostensibly to protect US interests in Baghdad and Irbil.

Obama has ruled out putting US combat troops on the ground, although senior military figures have argued that air strikes will be insufficient to stop Isis. Retired general Anthony Zinni has advocated the deployment of two US brigades.

Obama's speech follows weeks of discussions and intense lobbying of allies by the president at last week's Nato meeting in Wales. 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 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. There seems to be greater acceptance among Americans for a more aggressive military posture in the Middle East. Two polls released on Tuesday confirmed the shift in public mood following the beheading by Isis of two American journalists, James Foley and Steven Sotloff.

A Washington Post/ABC News survey found that 65% of Americans support air strikes 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.

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.