The U.S. State Department accused the Chinese government on Friday of conducting a secret nuclear test in 2020, while the Trump administration is seeking to include Beijing in new nuclear arms control agreements.

The new accusation, which Undersecretary of State for Arms Control and International Security Thomas DiNanno told a United Nations Disarmament Conference in Geneva, came the same week as the last remaining U.S.-Russia nuclear arms control treaty expired without an extension. The allegation also underscores the Trump administration’s view that a new nuclear agreement must include Beijing, the rapidly growing nuclear power, for it to be worthwhile.