The U.S.-led coalition could take up to two years to expel the Islamic State group from Iraq, and Baghdad's own forces will be incapable of proper combat operations for months, Britain's foreign minister warned on Thursday.

Speaking before he hosted a meeting of 21 coalition members in London, Foreign Secretary Philip Hammond said the task of pushing the Islamist militants back would be slow.

"This isn't going to be done in three months or six months. It's going to take a year, two years to push ISIL (the Islamic State group) back out of Iraq but we are doing the things that need to be done in order to turn the tide," Hammond told Sky News.