SEOUL (AFP/REUTERS) - North Korea on Thursday (Feb 11) ordered all South Koreans to leave the jointly run Kaesong industrial zone immediately and announced it was seizing all materials left in the complex.

North Korea also said it was freezing the assets of companies operating there, calling the South's decision to suspend operations at the zone a "declaration of war".

In a statement published by the official KCNA news agency, the committee said it was closing Kaesong and declaring it a military area.

It also announced the cutting of all military communications with South Korea, including the main channel through the border truce village of Panmunjom, as soon as the expulsion of South Korean citizens had taken place.

It did not specify how long the links would be cut for.

"South Korean enemy forces will experience themselves the harsh and painful price they should pay for halting the Kaesong industrial complex," it said.

All South Koreans were ordered to leave Kaesong by 5pm Pyongyang time and told they could take nothing but their personal possessions.

"We seize all assets of the South Korean companies and related organisations including machinery, raw materials and goods," the statement said.

The order to leave immediately was published on KCNA just 30 minutes before the expulsion deadline.

But all the South Koreans expelled by North Korea from the jointly-run Kaesong industrial complex on Thursday have safely crossed the border, Seoul's Unification Ministry said.

The 280 citizens in Kaesong at the time the expulsion order was issued crossed back into South Korea shortly before 10pm local time, the ministry said in a statement.

Despite the ban on removing anything beyond their personal belongings, some trucks crossing the border after the expulsion order were carrying factory materials.

"No one stopped us when we were moving our goods into the truck," said Mr Park Seung Gul, the manager at a textile company in Kaesong.

The announcement by the Committee for the Peaceful Reunification of Korea (CPRK) followed Seoul's decision on Wednesday to shut down Kaesong operations as punishment for the North's recent nuclear test and long-range rocket launch.

The CPRK statement did not specify what might happen to anyone failing to make the expulsion deadline.

The measures mark a significant escalation of cross-border tensions that have been elevated since the nuclear test last month and a long-range rocket launch on Sunday.

South Korea on Wednesday gave South Koreans until Saturday to quit the complex. It also announced on Wednesday it was closing down operations at Kaesong, which lies 10km inside North Korea, in protest over the North's nuclear test and rocket launch.

The owners of the 124 South Korean companies operating factories in Kaesong had sent hundreds of empty trucks into the North on Thursday morning to start loading up goods and equipment.

"We will make the utmost efforts to make sure that all our nationals return home safely," Seoul's Unification Ministry said in a statement.

Several people who crossed back into the South on Thursday morning said they had noticed an increased military presence in Kaesong, including armed soldiers carrying backpacks and sleeping bags.

Ms Kim Soo Hee, a nurse working at a medical clinic in the complex, she had seen several military trucks arriving in Kaesong and a number of armed soldiers with backpacks and sleeping bags.

"There were more soldiers around the complex than usual," she said.

Relations between the two Koreas have always been volatile, but analysts said the current situation risked turning into a full-blown crisis.

"Now we can say that all strings between the Koreas have been cut and that there are no more buffers," said Dr Ko Yoo Hwan, a professor of North Korean Studies at Dongguk University in Seoul.

"An escalation of tensions is inevitable, and I see further trouble ahead with Kaesong and the issues of seized assets, especially if North Korea militarises the zone," Dr Ko said.

In September 2014, Pyongyang drafted a new operational regulation - rejected by Seoul - that would have allowed the North to detain South Korean businessmen in Kaesong in the event of an unresolved business dispute.

Defending its decision to halt operations at Kaesong on Thursday, Seoul said North Korea had been using the hundreds of millions of dollars in hard-currency that it earned from Kaesong to fund its nuclear weapons and ballistic missile programmes.

The government's move was slammed as "utterly incomprehensible" by the Kaesong company owners who said their businesses were being destroyed by politics.

Born out of the "sunshine" reconciliation policy of the late 1990s, Kaesong opened in 2004 and proved remarkably resilient, riding out repeated crises that ended every other facet of inter-Korean cooperation.

Earlier in the day, the United States also signalled unilateral moves against North Korea, with the US Senate unanimously adopting a Bill expanding sanctions.

The United States and its main Asian allies, South Korea and Japan, have led a push for tough UN Security Council sanctions, but have met resistance from North Korea's main diplomatic protector China.

Although fiercely critical of Pyongyang's nuclear ambitions, Beijing is more concerned at the prospect of the North Korean regime being pushed to collapse - triggering chaos on China's border.

The Senate Bill would punish any person or entity importing goods, technology or training related to weapons of mass destruction, or engaging in human rights abuses.

Penalties would include the seizure of assets, visa bans and denial of government contracts.

It also aims to cut down on money laundering and narcotics trafficking - two major illicit activities believed to be funnelling millions of dollars into leader Kim Jong Un's inner circle.