Three North Korean women who worked at a state-run restaurant in China have arrived in South Korea after defecting, authorities in Seoul said Wednesday.

It was the second such group defection announced this year, after 13 female employees of another Pyongyang-operated restaurant in China arrived in Seoul in April.

The South Korean Unification Ministry confirmed the arrival of the three, but declined to give any details of how they managed to get to Seoul.

According to unidentified government sources cited by the Yonhap news agency, they arrived via Thailand, after running away from their restaurant in the northern Chinese province of Shanxi.

Their case will fuel tensions with North Korea which insists the previous group of 13 women had been tricked into defecting and were being held in Seoul against their will.

Reports of the latest defection first emerged in late May, several weeks after the three women had initially escaped.

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

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 an even more important source of income than before.

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

Since the group defection in April, North Korean state media has repeatedly run emotional interviews with the 13 women's relatives still in North Korea, demanding their immediate return.

South Korea has rejected the North's "kidnapping" claims and refused Pyongyang's demands to allow the women's parents to travel to Seoul to meet their daughters.

Nearly 30,000 North Koreans have fled poverty and repression at home to settle in the capitalist South.

But group defections are rare, especially by staff who work in the North Korea-themed restaurants overseas and who are handpicked from families considered "loyal" to the regime.