A U.S. military drone flew over an inter-Korean border region during combined drills with South Korea on Thursday, military officials said, prompting residents in the area to evacuate as a precaution.

The unmanned flying vehicle was detected south of a general outpost in northern Gyeonggi Province near the border with North Korea and was later identified as a U.S. military drone belonging to its Marine Corps stationed here, according to military officials from both sides.

"Upon further verification, the unmanned vehicle was identified as a U.S. military drone participating in a training exercise," an official from South Korea's Joint Chiefs of Staff (JCS) told reporters.

The JCS official said operation measures were carried out in line with standard protocol.

The U.S. Forces Korea (USFK) also confirmed it was one of its drones and said it was working with South Korean authorities to assess the situation.

"This incident occurred during bilateral training between U.S. Marines and the Republic of Korea Marine Corps' 1st Marine Division," the USFK said, referring to South Korea by its official name.

"U.S. Marine Corps personnel are assessing the circumstances surrounding the incident, and continue to coordinate with our Republic of Korea Marine Corps partners and appropriate local authorities," it added.

Earlier in the day, an alert was issued over Cheorwon, about 70 kilometers north of Seoul, near the buffer zone along the inter-Korean border, advising residents in the area to take shelter as the drone flight was detected.

The South Korean military first detected the drone through its surveillance assets, but it was apparently unable to immediately determine if the drone could have come from North Korea.

Based on how the drone flew, its planned flight may not have been shared with South Korean military units in the region, or the drone may have veered off its intended course.

The JCS official declined to give further details, such as whether the drone flew over the no-fly zone under the now-suspended 2018 inter-Korean military pact with North Korea.

The no-fly zone was established under the administration of former liberal President Moon Jae-in to prevent unintended military clashes with Pyongyang along the tensely fortified border.

It prohibits the operation of planes and drones within 15 km of the Demilitarized Zone that separates the two Koreas in eastern areas and 10 km in western regions. But the pact was suspended amid soured inter-Korean ties.

The military said earlier it was in the process of gathering more information about the U.S. drone flight.