The Kaesong Industrial Complex, the last bastion of inter-Korean cooperation amid escalating military tension, is facing its worst crisis since the project started in 2003.

Pyongyang on Wednesday closed the border to South Korean workers and vehicles at the industrial park, prompting Seoul to warn that it could resort to military action should the safety of its citizens there be compromised.

But thirty-three South Koreans who work at the complex returned to the South in the afternoon. Some said North Koreans there looked unusually tense.

"Workers who normally wear civilian clothes wore military uniforms and patrols were strengthened," said one worker.

The North also replaced civilian customs staff with soldiers and customs checks took longer than usual. Another worker said that there were even armored vehicles on standby.

"The demeanor of North Korean workers appears to have changed overnight," said another. "They were colder than usual and didn't smile." And a fourth reported, "Inter-Korean relations are a problem, but we're very concerned that we could lose our business."

Some said their co-workers who remain in the complex wonder whether they would be allowed to return or be taken hostage. Around 800 South Korean workers remain.

"If parts or food fail to reach the North for two to three days, the assembly lines will stop running and workers will starve," said one worker.