The United States and its Asian allies tightened the economic screws on North Korea Thursday, with the US Senate adopting fresh sanctions and South Korean firms abandoning a joint industrial park that helped fund Pyongyang's nuclear weapons programme.

The unilateral moves, which included Japanese sanctions, came with UN Security Council members still stalled on how far to go in punishing the North for its latest nuclear test and long-range rocket launch.

Following Seoul's surprise decision to shut down the Kaesong industrial zone in North Korea, hundreds of South Korean trucks crossed the border Thursday morning to retrieve finished goods and equipment from the factories there.

Defending what it called an "unavoidable" decision to close the jointly run park, 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 move was slammed as "utterly incomprehensible" by owners of the 124 South Korean companies operating factories in the estate, who said their businesses were being destroyed by politics.

"I'm speechless at what has happened," said Jang Ik-Ho, a manager with an engineering company in the complex.

"The companies have all done our best to make things work, and now this happens. What did we do to deserve this?" Jang said, as he prepared to cross into the North.

Seoul has called on Pyongyang to ensure the "safe return of our citizens" amid concerns that the North Korean authorities might refuse to let everyone leave the park, which lies 10 kilometres (six miles) over the border.

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.

- Safety concerns -

"It would be a lie to say I'm not worried about my personal safety," said one textile company operative, Yoon Sang-Young.

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

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.

The only exception was in 2013 during a period of heightened cross-border tensions when Pyongyang effectively shut down the zone for five months by withdrawing its 53,000 workers.

There has so far been no official reaction from Pyongyang to the shutdown.

US Assistant Secretary of State Daniel Russel welcomed Seoul's initiative.

"More steps are needed to convince the (North Korean) leadership that it is not going to be possible to have access to the international economic system let alone economic or financial aid as long as North Korea continues to pursue nuclear and missile programmes," Russel said.

- New US sanctions -

The US Senate later voted unanimously to adopt a bill expanding sanctions.

The measure, which must be reconciled with a similar House version that passed last month, 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.

"This dictatorial regime must learn that its actions have consequences," said House Speaker Paul Ryan.

Japan also unveiled unilateral measures on Wednesday, including prohibiting North Korean ships from entering Japanese ports and a total entry ban on North Korean nationals into Japan.

North Korea's main diplomatic protector, China, has been resisting the US-led push for tougher UN sanctions.

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