Efforts to relocate a US base on Japan's Okinawa appeared to suffer a new setback Sunday, 17 years after they began, with the reported electoral victory of an opponent of the project.

The mayor of the town of Nago on the east coast of Okinawa has won re-election, according to the TBS news station after the majority of votes were counted.

Susumu Inamine, supported by several leftist parties, is a strong opponent of the joint project by the US and Japanese governments to move the US Marines' Futenma Air Station, sited in an urban area in the south of Okinawa, to Nago bay.

The mayoral race pitted Inamine against Bunshin Suematsu, who supports the move.

Last month, more than 17 years after Washington and Tokyo agreed to move the base from the densely populated urban area, the Okinawa government finally consented to a landfill that will enable new facilities to be built on the coast at Nago.

The issue had been deadlocked for years, with huge opposition to any new base among Okinawans fed up with playing host to an outsized share of the US military presence in Japan.

The mayoral election was being closely watched by both Japan and the United States amid concerns an Inamine victory could further postpone the relocation.

The mayor of Nago does not have the right to overthrow plans to relocate the base but could refuse to approve the use of roads and other facilities necessary for building works.

The agreement reached in December was hailed as a breakthrough that could remove a running sore in relations between the two allies.

Okinawa's Governor Hirokazu Nakaima, long a thorn in the central government's side, gave the plan his approval after Japanese Prime Minister Shinzo Abe promised Okinawa financial aid of at least 300 billion yen ($2.9 billion) every year until fiscal 2021.

Japan and the United States agreed the relocation plan in 1996 but it never went ahead because of opposition from many residents of Okinawa, which hosts the bulk of some 47,000 US troops based in Japan.

Opponents support the removal of the US base from the town of Ginowan but want it relocated out of Okinawa altogether.