TOKYO, Jan. 9 (Xinhua) -- Japan lodged an official protest with the United States on Tuesday regarding two emergency landings made by U.S. military helicopters in Okinawa Prefecture as locals' anger and fear at the frequency of U.S. military-linked accidents continues to rise.

Japanese Foreign Minister Taro Kono told U.S. Ambassador to Japan William Hagerty during talks over the phone that the frequency of U.S. military-linked accidents is causing increasing anxiety to Okinawans.

The Japanese government has grave concerns about the frequency of such accidents and urged the U.S. side to take fundamental measures to prevent their recurrence, he said.

U.S. Defense Secretary Jim Mattis earlier in the day apologized for the recent mishaps involving the military helicopters in Okinawa, while Japan's Defense Minister Itsunori Onodera urged Mattis in a telephone conversation to ensure effective measures are taken to prevent such accidents happening again.

Onodera said the U.S. side must carry out thorough inspections and maintenance of its aircraft, with Mattis promising to deal with the issue, that has sparked vociferous indignation from Okinawans, seriously.

The government's protest on Tuesday follows an AH-1 attack helicopter belonging to the U.S. Marine Corps Air Station Futenma making an emergency landing at a waste disposal site in the village of Yomitan in Japan's southernmost prefecture on Monday.

According to local police and the U.S. military, there were no reports of injury to persons or property.

Monday's incident follows a UH-1 heavy-lift transport helicopter on Saturday making an emergency landing on a sandy beach on Ikei Island, also in Okinawa.

The U.S. Marine Corps said the emergency landing on the small islet was due to "indications of the main rotor moving at too high a speed."

The UH-1 transport helicopter touched down just 100 meters away from a residential house.

Both helicopters are based at the controversial U.S. Marine Corps Air Station Futenma, which is central to a protracted row between the local and central government about its relocation from the densely-populated Ginowan district, to the coastal Henoko region also on the island.

Okinawa Governor Takeshi Onaga, a staunch opponent to the central government's plan to relocate the Futenma base and proponent for reducing the U.S. military footprint in Okinawa, was quoted by local media as telling reporters Tuesday: "I am really speechless. Nothing has moved forward to prevent such incidents happening."

Local residents have also been quick to voice their indignation and concerns, too.

"I was shocked to see the U.S. attack helicopter had landed so close to my house," one resident living nearby was quoted as telling local media on Monday.

"Although the helicopter didn't appear to be damaged or on fire, I was very afraid, particularly as a similar incident took place just two days ago. 'Not again,' I thought to myself."

"While I am angry, I am also afraid and concerned whether the U.S. military is seriously trying to improve its safety standards," the 33-year-old local man said, adding that it might only be a matter of time until one of the helicopters crash-lands onto his house.