SEOUL (REUTERS) - South Korea's presidential office on Friday (Aug 6) dismissed opposition lawmakers' calls for explanations on a relationship between President Moon Jae-in and three activists who were arrested on charges of coordinating with North Korean agents.

The South Korean activists were arrested on Monday on charges of violating the National Security Act by campaigning, under an order from a North Korean operative, against the South Korean military's plans to buy US stealth fighters, according to police.

Under usual procedure in South Korea, authorities have not identified the activists nor has the identity of their lawyers been disclosed.

Police and the National Intelligence Service (NIS) have also accused the three of receiving some US$20,000 (S$27,025) from the operative.

Police said the activists had met the operative in the Chinese city of Shenyang near the North Korean border numerous times since 2017.

Their mission included "underground organisations" to carry out pro-North Korean, anti-US movements, a police official said, declining to elaborate citing the investigation.

The Korean peninsula has been divided for decades and spy scares and scandals between the Cold War rivals have been common over the years.

The latest case sparked political uproar when the Chosun Ilbo newspaper, citing unidentified sources, said the activists had worked on Mr Moon's presidential campaign as special advisers on labour issues before he took office in 2017.

The Chosun Ilbo and other media outlets also said the activists had met a senior lawmaker of Mr Moon's ruling Democratic Party to discuss possible projects with North Korea.

Police, the NIS and the Cheongju District Court, which issued an arrest warrant for the activists, could not be reached for comment on the Chosun Ilbo report.

The main opposition People Power Party described the case an "espionage scandal" and demanded that Mr Moon and the Democratic Party clarify their relations with the activists.

A spokesperson at Mr Moon's office dismissed the assertion that the activists had worked for Mr Moon, saying in a statement it was "not worth mentioning." A spokeswoman at the Democratic Party said it had no comment.

The liberal Mr Moon has been keen to improve relations with North Korea, saying both sides would benefit economically, despite its nuclear weapons and missile programmes.

In the first sign of easing of tension for months, the two Koreas last week restored hotlines which North Korea cut a year ago as ties came under stress.