SEOUL (AFP) - Thirteen North Koreans working in state-run restaurants outside the country have defected, a South Korean government official said on Friday (April 8).

The defectors, one man and a dozen women, arrived in the South on Thursday, Unification Ministry spokesman Jeong Joon Hee told reporters in Seoul.

He declined to identify the country where the restaurant was located.

The South Korean government estimates that Pyongyang rakes in around US$10 million every year from some 130 restaurants it operates - with mostly North Korean staff - in 12 countries.

Last month, while unveiling a series of unilateral sanctions on Pyongyang over its January nuclear test, Seoul had urged South Korean citizens overseas to boycott any such establishments, saying their profits funded the North's nuclear weapons programme.

There have been defections by individual restaurant workers in the past, but this is the first time the entire staff of one restaurant has defected en masse, Mr Jeong said.

He quoted one of the defectors as saying that everyone had been "on the same page" about escaping to South Korea.

"Medical check-ups show that their health condition is relatively good," Mr Jeong said.

"We cannot reveal the country and the route they took.

"That's because we are concerned about a possible diplomatic clash with a third country, the protection of the group and other possible cases that might arise in the future," he added.

North Korea is known to be strict about vetting and selecting people to work in its overseas restaurants, knowing they will inevitably be exposed to information about the outside world that they are mostly protected from in the North.

"From what we know, there is some competition for being deployed to overseas restaurants. We believe that in order to win the competition, they must be from relatively good families," Mr Jeong said.

Tough United Nations sanctions imposed on North Korea after its January nuclear test significantly curtailed the isolated state's ability to earn hard currency, making the restaurants even more important than before as an income source.

There have been reports of staff not being paid, as the restaurants were pressured into increasing their regular remittances to Pyongyang.

"We believe so," Mr Jeong said, when asked if the UN sanctions had been one trigger for the staff to plot an escape to South Korea.

The defections come at a time of elevated military tensions on the divided Korean Peninsula. North Korea has condemned Seoul and Washington for spearheading the sanctions drive at the UN, while also lashing out at annual, large-scale military war games that South Korea and the United States kicked off last month.

In recent weeks, the North's state media has carried regular threats of pre-emptive nuclear strikes against both countries. Mr Jeong said the South Korean government had decided to go public with the group defection because it was such a rare case.

"Medical check-ups show that their health condition is relatively good," he added.

Overseas labour is a large source of foreign currency for North Korea, which earns between US$200 million and US$300 million from approximately 50,000 workers per year, according to the Seoul-based Database Centre for North Korean Human Rights.

The lion's share work in China and Russia.