Tensions have soared on the Korean Peninsula since December last year, when the North test-launched a long-range rocket.
In February, it conducted its third nuclear test and drew fresh UN sanctions.
The North also warned this week it would reopen its mothballed Yongbyon reactor — its source of weapons-grade plutonium that was closed in 2007 under an aid-for-disarmament accord.
On Thursday, North Korea blocked access to its Kaesong joint industrial zone with South Korea for the second day running, and threatened to pull out its 53,000 workers in a furious reaction to the South’s airing of a “military” contingency plan to protect its own workers there.
The Unification Ministry said there were still 608 South Korean citizens in Kaesong, which was shut Friday for a scheduled North Korean holiday.