A trade fair between South Korean and Chinese small and mid-sized firms in the Chinese city of Dandong on the border with North Korea was canceled days before its opening because of unspecified safety concerns, a diplomatic source close to the matter said Friday.

   Organized by several South Korean municipal governments and the Chinese city of Dandong, which faces North Korea across the Amnok River, the five-day trade fair was previously scheduled to begin next Thursday.

   About 150 firms from South Korea and another 150 from China had planned to take part in the trade show. The event, if held, would have been the first of its kind, whereas North Korea and China have been holding such events annually since 2012.

   "The Dandong city government notified the South Korean side that the trade fair was canceled because of safety worries," the source said on the condition of anonymity. The participants were reportedly told that it was a decision made by the Chinese central government.

   It was not immediately known whether the "safety worries" were related to North Korea.

   However, the South Korean government has issued a travel advisory warning its citizens to be cautious in visiting China and its border regions with North Korea.

   The travel warning came as North Korea has issued threats after North Korean restaurant workers in China have defected to South Korea.

   Three North Koreans who had worked at a North Korean-run restaurant in China arrived in South Korea on Wednesday after escaping their workplace last month, Seoul officials said.

   It marked the second group defection this year after 13 North Korean restaurant workers in China defected to South Korea in April.

   Also Wednesday, a North Korean delegation led by Ri Su-yong, vice chairman of the North Korean ruling party's central committee, returned home, wrapping up their three-day visit to Beijing.

   Ri's visit to China is seen as North Korea's attempt to mend frayed ties with China over Pyongyang's nuclear weapons program.

   China, North Korea's traditional ally and economic lifeline, has joined tougher international sanctions aimed at punishing the North for its nuke and missile tests early this year.

   It marked the first visit to China by ranking North Korean officials since the communist country conducted its fourth nuclear test in January and launched a long-range rocket the following month. (Yonhap)