WASHINGTON -- Pentagon leaders are trying to "fine tune" U.S. strategy for ousting the Islamic State group from Iraq, focusing on faster and better training and arming of Sunni tribes whose combat role is central to reversing the extremists' advances, senior U.S. officials said Thursday.

Defense Secretary Ash Carter, speaking to reporters while traveling to Asia, said he told senior military officers at the Pentagon this week to come up with ideas to improve training and equipping, particularly of the Sunni tribes who complain the Shiite-dominated government in Baghdad is dragging its heels on helping them.

"I can't describe to you what the possibilities are, because folks are looking at them right now," Carter said.

The scramble for answers comes after Islamic State forces captured the Anbar province capital of Ramadi as Iraqi forces fled May 16.

Although the White House says those forces were not U.S.-trained, the defeat prompted Carter to say last weekend the Iraqis lacked "the will to fight."

President Barack Obama on Tuesday said it was time for the U.S. to consider whether it was delivering military aid to Iraq efficiently.

The U.S. military strategy in Iraq is built on airstrikes to degrade the Islamic State forces while rebuilding Iraqi security forces to regain the vast swaths of territory in the north and west that were lost over the past 18 months.