An Air Force investigation team said Thursday that this week's unintentional launch, and mid-air explosion, of an anti-aircraft guided missile was caused by a mistake during a routine maintenance check.

The Cheongung medium-range surface-to-air missile exploded in midair near an Air Force base in Chuncheon, 85 kilometers northeast of Seoul, on Monday after being unintentionally fired off. No one was injured in the blast.

The Air Force said that two mechanics mistakenly left an operational cable connected to the missile during the maintenance check, although they were supposed to replace that with a testing cable.

The missile was unintentionally launched when the operational cable received a mock launch signal for a testing purpose. The mechanics admitted their mistake and will be referred to a disciplinary panel, the armed service said.

"As the mechanics failed to have clear communication between them, they carried out a maintenance check with the operational cable still connected to the missile," the Air Force said in a press release.

"We feel sorry to citizens for causing concerns and will do our best to prevent a recurrence of a similar case," it added.

The Cheongung, worth 1.5 billion won ($1.33 million) each, is to strike a hostile aircraft at an altitude of around 40 kilometers. In November 2017, two Cheongung missiles flew around 40 kilometers and hit a mock target accurately in its first live-fire mission. (Yonhap)