Four North Korean banks that were still able to use the Swift money-transfer messaging system despite U.S. Treasury sanctions will be deactivated from the network in coming days, leaving none of the country’s banks with access.

Swift spokeswoman Natasha de Teran told The Wall Street Journal in a statement Thursday that the four North Korean banks “will no longer have access to the Swift” service. She said that, “given the increased ongoing international attention” on Pyongyang, Brussels-based Swift had informed Belgian and...