Media playback is unsupported on your device Media caption Obama: "This resolution reflects our core objective to destroy Isil"

President Barack Obama has said Islamic State (IS) militants "will lose" as he asks Congress to formally authorise military force against them.

"Our coalition is on the offensive. Isil [IS] is on the defensive, and Isil is going to lose," he said.

He warned against dragging the US into another prolonged ground war and called on lawmakers to show a united front.

The US has carried out air strikes against IS since last year, in a coalition with other nations.

It's the first time a US president has sought Congressional approval to use military force since President George W Bush in 2002, ahead of the invasion of Iraq.

In a White House address, he said the draft authorisation did not change the mission's core objective to "degrade and destroy" IS.

Image copyright Reuters Image caption Islamic State has its stronghold in Raqqa, Syria

Flanked by the Vice-President Joe Biden, and the US secretaries of state and defence at the White House, Mr Obama said this did not signal an "endless war".

He said he was "convinced that the United States should not get dragged back into another prolonged ground war in the Middle East" but said fighting IS would take "some time".

Backing from Congress would send a powerful message that the US was united in its opposition, he said.

"Make no mistake. This is a difficult mission," Mr Obama said. "But our coalition is on the offensive. ISIL is on the defensive, and ISIL is going to lose."

Wednesday's draft proposal would repeal Mr Bush's 2002 Authorization for Use of Military Force (AUMF) but would leave in place the 2001 resolution that was approved after the 9/11 terror attacks in order to search for al-Qaeda operatives.

Analysis - Tara McKelvey, BBC News White House reporter

Political cover plays a role here. Liberals have been upset about the Authorization for Use of Military Force (AUMF) because President Barack Obama has used it to justify drone strikes and the killing of an American.

White House officials don't like being tied in any way to the Bush administration and neither do Democrats who will run for office in 2016.

So seeking a new AUMF is a way for Mr Obama and other Democrats to distance themselves from former President George W Bush.

The new AUMF is also a way for Mr Obama to get Republicans behind him in the war against IS.

The new measure is expected to set up a debate about America's role in the Middle East and how to best counter the militant group.

In a letter sent to Congress along with his draft, Mr Obama said while "existing statutes" allow the air strikes, he has "repeatedly expressed my commitment to working with the Congress to pass a bipartisan authorization for the use of military force (AUMF) against ISIL".

The plan could include limited ground operations by US forces in special circumstances.

The issue of ground forces, even limited, is expected to make the legislation harder to pass.

Some Republicans are in favour of using ground troops but many in Mr Obama's own party oppose it.

On Wednesday, the top Democrat in the House of Representatives, Nancy Pelosi, issued a statement calling for "narrowly tailored" legislation for the fight against IS but did not endorse Mr Obama's proposal.