U.S. President Barack Obama's pick to run the Pentagon faces a Senate confirmation hearing on February 4.
Ashton Carter is expected to face questions from the Senate Armed Services Committee on Islamic State militants, the situation in Afghanistan, Russia's interference in Ukraine, and Iran's nuclear program.
In response to written questions from the committee, Carter said he might consider reviewing the schedule for withdrawing U.S. troops from Afghanistan by the end of 2016 if security conditions there worsen.
About 10,600 U.S. troops remain in Afghanistan after most foreign combat troops withdrew at the end of 2014.
Carter expressed interest in expanding counterterrorism cooperation with Pakistan and advancing efforts to integrate Sunni tribal forces into Iraq's battle against IS militants.
He also said he would work with coalition partners to ensure that IS militants don't expand into Afghanistan.
Carter served as deputy defense secretary from 2011 to 2013.
If confirmed, he would replace Chuck Hagel as Pentagon chief.