ABOARD A US MILITARY AIRCRAFT (REUTERS) - Defense Secretary Ash Carter said on Thursday Pentagon officials have begun to examine how the United States military could better equip and train Iraqi troops after the recent fall of the Iraqi city of Ramadi to Islamic State in Iraq and Syria (ISIS) insurgents.

Carter told reporters on his plane to Asia that he had convened a group of defense policy officials and military officers from US Central Command and the Pentagon's Joint Staff to look at how "we can enhance, hasten" the mission to train and equip Iraqi forces.

The initial meeting took place on Tuesday before Carter departed on a trip to Asia. "The events of recent weeks there (in Iraq) have highlighted the central importance of having a capable ground partner and that's what the purpose of our train-and-equip program is. So we are looking," Carter said.

Ramadi, capital of the predominantly Sunni western Anbar province, fell to Islamic State militants last week after Iraqi security forces withdrew.

The capture of the city was the militants' biggest victory since US and coalition forces commenced an air campaign last August to help Iraqi security forces halt the advance of the group, which has seized control of parts of Syria and Iraq.

Carter told CNN's "State of the Union" program on Saturday that Iraqi troops vastly outnumbered Islamic State fighters and the city fell after "Iraqi forces just showed no will to fight."

Iraqi Prime Minister Haidar al-Abadi rebutted Carter's characterisation and US President Joe Biden called the Iraqi leader to reassure him of American support, a move seen as damage control.

But a Pentagon official said Carter stood by his comments.

Carter told reporters aboard the plane that it was important to review the US training effort in light of Ramadi to see"what we can do to enhance the effectiveness."

"I think training and equipment affect the effectiveness of the forces and therefore ... their confidence in their ability to operate, so there is a direct relationship," he said.

Carter said it was "extremely important" to involve Anbar's Sunni tribes in the fight.

A senior defense official said later Carter was not considering directly arming Sunni tribes, which would be a major shift in policy.

Current US strategy calls for providing any weapons directly to the Shi'ite-led government in Baghdad, which in turn would distribute them to Sunni tribes and Kurds.