China is investigating executives of a North Korean bank believed to finance the illicit procurement of arms and materials related to the isolated country's banned nuclear program, South Korea's JoongAng Daily reported on Monday.

China and the United States have agreed to step up cooperation in the U.N. Security Council and in law enforcement channels after North Korea's fifth nuclear test on Sept. 9, the White House said last week.

While China is North Korea's sole major ally, it disapproves of its nuclear and missile programs.