PAJU, South Korea — North Korea today barred South Korean workers from entering a jointly run factory park just over the heavily armed border in the North, officials in Seoul said, a day after Pyongyang announced it would restart its long-shuttered plutonium reactor and increase production of nuclear weapons material.

The move to block South Koreans from going to their jobs at the Kaesong industrial complex, the last remaining symbol of detente between the rivals, comes amid increasing hostility from Pyongyang, which has threatened to stage nuclear and missile strikes on Seoul and Washington and has said the armistice ending the 1950s Korean War is void.

Seoul’s Unification Ministry spokesman Kim Hyung-suk said Pyongyang was allowing South Koreans to return home from Kaesong. Three workers returned this morning; dozens more were scheduled to return later. But Kim said about 480 South Koreans who had planned to travel to the park were being refused entry.

North Korean authorities cited recent political circumstances on the Korean Peninsula when they delivered their decision to block South Korean workers from entering Kaesong, Kim said without elaborating.