Defeating IS may take longer than three years: Pentagon

US Defence Secretary and Pentagon chief Ashton Carter said on Wednesday that the fight against the Islamic State (IS) terrorist group may take longer than the three year timeline stipulated in President Barack Obama's force authorisation request.

"I cannot tell you that our campaign to defeat ISIL (IS) would be completed in three years," Carter told the US Senate Foreign Relations Committee during a hearing on Obama's war authorisation request to the Congress, according to a Xinhua report.

On February 11, the Obama administration unveiled its war authorisation bill against the IS that would prohibit the use of "enduring offensive ground forces" and limit engagement to three years.

Carter said that although he was uncertain if the task could be finished within three years, the provision was "sensible and principled".

"The president's proposed authorisation affords the American people the chance to assess our progress in three years' time, and provides the next president and the next Congress the opportunity to reauthorise it, if they find it necessary," he said.

Under the authorisation for the use of force passed in 2001, the Obama administration could use force against the extremist group Al Qaeda and its affiliates -- the IS in this case -- without permission from the Congress.

US Secretary of State John Kerry said that the purpose of seeking a formal force authorisation was to highlight a united America.

"A clear and careful expression of this Congress' backing at this point and time would expel doubt that might exist anywhere that Americans are united in this effort," Kerry said.

However, the Obama administration's war authorisation pitch is expected to undergo major changes in its language, as hawkish Republicans said that Obama's bill would constrain the military, while the Democrats demanded a clearer language in prohibiting a large US ground combat presence. The worst scenario would be inaction by the Congress.

During the hearing session, Kerry was interrupted several times by anti-war protesters who yelled repeatedly that the US campaign against the IS would lead to the death of "innocent people".