North Korea on Wednesday banned South Korean workers from entering a joint industrial complex near the demilitarized border, officials in Seoul said, putting in jeopardy a project that provides the only daily contact between the two nations.

The North did not say how long the entry ban would last, and it did allow South Korean workers who had been at the facility to return home.

The move is significant because the Kaesong Industrial Complex has long stood as a near-untouchable symbol of cooperation on the peninsula, operating even during a pair of fatal 2010 attacks launched by the North on the South. The complex, where South Korean firms use North Korean labor, is a key source of revenue and foreign currency for the government in Pyongyang.

A spokesman for the South Korean Ministry of Unification said his government demanded the entry ban be “lifted immediately,” according to the South’s Yonhap news agency.

The North banned entry similarly at least once before, for a matter of days in 2009 during joint military exercises between the United States and South Korea. But this time, relations between the two Koreas are more tense, with Pyongyang recently declaring a “state of war” and vowing to restart key nuclear facilities to produce fissile material for weapons.

Kaesong, located six miles north of the heavily fortified border, employs about 50,000 North Koreans and is home to about 120 South Korean businesses. Those businesses received tax benefits and low-interest loans from the South Korean government, as well as risk insurance.