The western crossing at Dorasan was not completely shut under the restrictions, but it failed to open at its usual time on Monday morning.

Hundreds of South Koreans were expelled from a joint industrial zone in the North as part of the restrictions.

Tourism trips and a cargo train to North Korea were suspended last week.

South Korea's Unification Ministry spokesman Kim Ho-nyeon said: "It is very regrettable that North Korea has imposed restrictions on border crossings.

"The North's measure should be immediately withdrawn."

A South Korean official said as many as 1,700 South Korean managers could return to the Kaesong industrial zone from Monday, Reuters news agency reported.

They would be "necessary personnel" needed to keep it running, and a reduction from almost 4,200 South Korean managers who had previously been allowed to enter.

South Korea has funded the Kaesong industrial complex just over the border in the North, and a ban on border crossings would make it very difficult for the plant to continue operating.

Some 30,000 North Korean workers are employed by South Korean companies at the complex, and jobs there are highly prized.

Relations between the Koreas have become increasingly strained since February when conservative President Lee Myung-bak took office in Seoul, pledging to get tough with Pyongyang over its nuclear weapons programme.

Last week South Korean activists sent thousands of propaganda leaflets into North Korea, ignoring threats from the North to sever relations.

Activists launched 10 huge helium balloons, each stuffed with 10,000 flyers critical of North Korean leader Kim Jong-il.

BBC