US President Barack Obama has approved sending up to 1,500 additional troops to Iraq, roughly doubling the number of US forces on the ground helping Iraqi and Kurdish forces battle the militant group Islamic State, US officials said on Friday.

Obama's decision greatly expands the scope of the US campaign and the geographic distribution of US forces, some of whom will head into Iraq's fiercely contested western Anbar province to act as advisers.

It also raises the stakes in Obama's first interactions with the US Congress after his Democratic Party was thumped by Republicans in midterm elections this week.