Former Pentagon official Ashton Carter emerged on Tuesday as the expected nominee to replace Chuck Hagel as U.S. defense secretary, sources said.

Carter, a former deputy secretary at the Department of Defense, would have the task of breaking into the tight-knit White House inner circle that President Barack Obama has leaned on to run national security policy.

His influence would be tested as the United States wrestles with a growing list of international crises — from the battle against the Islamic State in Iraq and Syria to troubled ties with Russia and a still-resilient Taliban enemy in Afghanistan.