The parents of 12 female North Korean staff who defected to South Korea last month while working at a restaurant run by the North in China have sent a letter to the United Nations asking the international body to mediate their return, a Chinese state-run media reported on Tuesday.

South Korea said on April 8 that the North Korean workers and their male North Korean manager defected to the South, in the biggest group defection since North Korean leader Kim Jong-un took power in late 2011.

North Korea has claimed the defectors were "abducted" by South Korean spies and proposed sending their family members to the South for meetings.

Seoul has rejected such accusatiosn and made clear that the 13 people defected on their own free will.

In the April 18 letter, the parents of the North Korean female workers urged the U.N. to help repatriate their loved ones, the state-run Global Times newspaper reported, citing the North Korean Embassy in Beijing.

The letter also accused South Korean intelligence authorities of "kidnapping" them, according to the report.

China's foreign ministry spokesman Lu Kang told reporters on April 11 that the 13 North Koreans left China using "valid passports."

About 29,000 North Koreans have defected to South Korea since the end of the 1950-53 Korean War, according to data by the South Korean government. (Yonhap)