Two U.S. Marines participating in parachute drop training on Tuesday landed in an undesignated field on Okinawa's Ie Island, the local village said.

On the same day, U.S. forces also conducted a parachute drop at Kadena Air Base on Okinawa Island, the fourth of its kind this year despite an agreement with Japan specifying that such training is to be carried out on Ie Island.

The parachute training at the Kadena base prompted Japanese Defense Minister Taro Kono to express strong concern. "It is a very regrettable development that could affect the Japan-U.S. alliance," he told reporters in Tokyo.

Since Japan and the United States agreed in 1996 to limit parachute drop training by U.S. forces to an airport facility on Ie Island, Kono has asked the United States to halt such training at the base on Okinawa's main island.

(Parachute drop training at Kadena Air Base on Okinawa Island in September 2017)

According to the Okinawa prefectural government, more than 20 U.S. servicemen had parachuted down to the Kadena base by 8 p.m. on Tuesday.

Although there were no reports of injuries or property damage from the latest training, it is likely to spark outcry in Okinawa due to past incidents.

In 1965, a girl died after being trapped by a trailer dropped in a parachute training at the village of Yomitan. At that time, Okinawa was under U.S. rule following Japan's surrender in World War II. Okinawa was returned to Japanese administration in 1972.

According to the village of Ie, two Marines landed in a field about 100 meters away from the designated airport facility at around 1:45 p.m.

The U.S. forces say their latest training was based on a bilateral accord enabling the Kadena base to be used for parachute training as an alternative when Ie Island is unavailable.

Concerns over the risk of base-related accidents and crime remain strong among people in Okinawa, which hosts the bulk of U.S. military facilities in Japan.

(Airport facility on Ie Island of Okinawa)