Five South Korean workers at a mining company in the Philippines were released after being detained by communist rebels for around six hours Monday.

The Foreign Ministry said a group of communist rebels ambushed the Japanese-owned mining company near Mindanao Island around 11 a.m., killing three Filipinos and detaining some 70 employees of the company, including the five South Koreans.

The ministry said it has yet to determine the reason the insurgents raided the mining firm and why they let the employees go after their detention.

Shortly after the incident occurred, the South Korean Embassy in Manila put an emergency countermeasures team into operation and confirmed the safety of the five South Koreans.