WASHINGTON (AFP) - The United States is sending about 100 commandos to Iraq to fight Islamic State in Iraq and Syria (ISIS) militants in that country and across the border in Syria, a US military spokesman said Wednesday.

"Probably around 100, maybe a little bit less," Colonel Steve Warren said.

"In fact, really fewer actual trigger-pullers, if you will ... It's a very small number, a double-digit number."

Warren's comments came the day after Defence Secretary Ashton Carter announced additional special forces troops would join the fight against the ISIS group, though the Pentagon chief did not give details on the size of the deployment.

Iraqi Prime Minister Haider al-Abadi reacted to Carter's announcement by saying Iraq does not need foreign ground troops to defeat the ISIS group, but he did not directly reject the deployment and US officials downplayed his remarks.

"We've been talking with the prime minister about this for weeks," Warren said.

The specially trained commandos will be involved in direct combat against ISIS extremists, but the Pentagon insists their mission doesn't contradict a White House pledge to avoid US "boots on the ground" and did not constitute "mission creep" in which the United States gets incrementally bogged down in a ground war against the ISIS group.

This is not "ground combat with armor and artillery and combined armed operations and death and destruction everywhere you look," Warren said.

"These are raids, these are a small number of highly skilled commandos conducting very precise, very limited operations... so there is a difference."

The United States already has about 3,500 troops in Iraq, but their mission is to "train and advise" local forces.