North Korea has closed the inter-Korean border for the third time in the last two weeks.

The Unification Ministry in Seoul said the North failed to approve border crossings on Friday, preventing about 660 South Koreans from traveling to Gaeseong Industrial Complex.

Ministry spokesman Lee Jong-ju said Seoul is urging Pyongyang through the industrial park management committee to issue immediate permission for cross-border passage, but the North is refusing.

The ministry said that if the North fails to approve the crossings, about 520 South Koreans scheduled to return on Friday afternoon will be stranded in the South-North joint industrial park.

South Korea and the U.S. wrapped up their annual Key Resolve joint military exercise on Friday, which was thought to be one of the main reasons for the North’s recent closures of the border.