Four S Koreans detained in North

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North Korea says it has detained four South Koreans on suspicion of entering the country illegally.

The four, who have not been identified, were "under investigation," the North's KCNA state media reported.

South Korea's Unification Ministry said it was trying to confirm the reports and trace about 1,000 of its citizens currently working north of the border.

The reports come a day after Pyongyang accused Seoul and the US of preparing for an attack.

The KCNA report did not say who the detainees were or when they had been arrested.

A spokesman for the South Korean Unification Ministry said they were "trying to confirm the safety and circumstances of our people who are currently located in North Korea".

The South's Yonhap news agency said there were 1,054 South Koreans working in North Korea on Friday, including 983 at the Kaesong shared industrial zone.

'Attack plan'

Earlier this month, Pyongyang released a US man of Korean descent who had crossed the border in late December.

North Korean authorities they had decided to "forgive and release" Robert Park, who entered the country over a frozen river carrying a letter for the North Korean leader, Kim Jong-il.

Pyongyang says it has since detained another American for a similar offence.

On Thursday, North Korea accused South Korea and the US of preparing to launch a attack.

The two countries are due to hold annual joint military exercises in early March.

Pyongyang said the exercises were "aimed at a surprise, pre-emptive attack" and warned it would respond to any such move by "mobilising all the offensive and defensive means, including nuclear deterrent".