U.S. President Barack Obama has warned of the growing threat of ISIS. Reuters

President Barack Obama has said that Islamist group ISIS represents a growing threat to the US.

In an interview with CBS News the president said Sunni militants pose a "medium- and long-term threat" to the US because of their extreme ideology.

"I think it's important for us to recognize that ISIS is just one of a number of organizations that we have to stay focused on," he explained.

"This is going to be a global challenge and one that the United States is going to have to address, but we're not going to be able to address it alone," Mr Obama added

"We're going to have to be vigilant generally. Right now the problem with ISIS is the fact that they're destabilising the country. That could spill over into some of our allies like Jordan."

Mr. Obama has rejected the idea of sending US troops to fight militant organizations around the world and instead is focused on providing training and resources to each nation's troops.

"... what we can't do is think that we're just going to play whack-a-mole and send US troops occupying various countries wherever these organizations pop up. We're going to have to have a more focused, more targeted strategy and we're going to have to partner and train local law enforcement and military to do their jobs as well."'

His comments come just hours after rebel fighters from ISIS captured four more towns in Iraq including a crossing on the border with Syria.

Haditha, Anah, Rawa, Rotba were taken as the (ISIS) swept east from the Syrian border, in its latest offensive.

ISIS had already taken control of the cities of Fallujah and Ramadi in predominantly Sunni Anbar province before it seized Iraq's second city Mosul, and Baiji, home to the country's largest oil refinery, in an aggressive offensive in the north.

While Foreign Secretary William Hague had earlier suggested Britain would provide humanitarian aid, cabinet minister Iain Duncan Smith has said the UK could provide logistical support if the US were to begin its own bombing raids.

"The Government has said it's not going to be doing any airstrikes or putting soldiers into Iraq but I think there are lots of other things we can do to help support them," he said.

He explained Britain could "make sure they get the right spare parts and support in maintaining those kind of aircraft and equipment and also support the Americans where they need it in terms of supply, et cetera."

Meanwhile, young Iraqis have been flocking to recruitment centres at the weekend to join the counter-offensive against ISIS. According to official records, some two million young men have volunteered in the past seven days.