WASHINGTON—North Korea has punched a hole in the web of United Nations sanctions intended to pressure Pyongyang to give up its nuclear-weapons programs and long-range missiles, accelerating its import of petroleum products through illicit ship-to-ship transfers and stepping up coal exports, according to a report to the U.N. Security Council by a panel of international experts that is expected to be issued this week.

Pyongyang has also defied sanctions by selling small arms and other military equipment to the Iran-backed Houthi...