US anti-IS mission is Operation Inherent Resolve

The decision was taken "a few days ago" by the chiefs of all the armed services, said Colonel Ed Thomas, spokesman for the US military's top officer, General Martin Dempsey.

"The operation is called 'Operation Inherent Resolve,'" Thomas told reporters Wednesday.

He offered no explanation as to why the Pentagon chose the name, which had already been cited in media reports as a possible choice that was rejected by some officers.

The US military usually announces the name of a military operation from the start.

The named operations also offer a way of organizing medals for service and valour, and that requirement might have provided the top brass an extra incentive to arrive at a decision.

The 2003 American invasion of Iraq was known as Operation Iraqi Freedom, and the war in Afghanistan is still known as Operation Enduring Freedom.

During the US occupation of Iraq from 2003 to 2011, there were more than 500 named operations, including Operation Airborne Dragon, Operation Soda Mountain and Operation Tapeworm.

Shortly after troops began arriving in Liberia last month to help in the effort against the Ebola outbreak, the Pentagon unveiled the name of that mission: Operation United Assistance.

But there had been no official moniker until now since US warplanes started bombing the IS group in Iraq in early August, prompting a spate of speculation and sarcasm.

Some commentators alleged that the lack of a name reflected the White House's lack of enthusiasm for intervening in Iraq and Syria. But US officials dismissed the idea as ridiculous.

Some reporters tweeted suggestions for possible names, including one favourite that referred to the American-made Humvee vehicles seized by IS militants - "Operation Hey That's My Humvee."