U.S. forces are helping Afghan troops respond to an attack at the American University of Afghanistan in Kabul on Wednesday.

A defense official said a "small number" of American advisers are assisting Afghan forces. "These advisers are not in a combat role; they are advising their Afghan counterparts," the official said.

About 9,800 U.S. troops are in Afghanistan. That number is expected to draw down to about 8,400 later this year and remain at that level through the end of Obama's presidency.

The attack at Kabul began on Wednesday morning and reportedly trapped several American professors and hundreds of students inside the university.

Witnesses took to Twitter, and Massoud Hossaini, a photographer for the Associated Press, said he heard shooting and an explosion.

The American University of Afghanistan is the country's first private, not-for-profit higher education university, and enrolled more than 1,000 students as of 2012.