A city mayor opposed to a plan to relocate a controversial U.S. airbase on Japan's Okinawa island was re-elected on Sunday, Kyodo news agency said, creating a political headache for Prime Minister Shinzo Abe and threatening friction with Washington.

Delays in relocating the U.S. Marines' Futenma air base, a move first agreed between Tokyo and Washington in 1996, have long been an irritant in U.S.-Japan ties and Abe is keen to make progress on the project as he seeks tighter ties with the United States in the face of an assertive China.

Abe's ties with Washington suffered after the United States expressed "disappointment" with his December 26 visit to Yasukuni Shrine, a pilgrimage that further strained relations with China and South Korea, which see the Tokyo shrine to Japan's war dead as a symbol of the country's past militarism.

Susumu Inamine - a staunch opponent of the relocation plan - was assured re-election as mayor of the Okinawa city of Nago, Kyodo said, citing projections shortly after the polls closed. An Okinawa newspaper also said Inamine's victory was certain.

Inamine has pledged to use his local authority block the relocation of the functions of Futenma from a populous part of central Okinawa to Nago's coastal Henoko area.

His main opponent had backed the plan and run with strong support from Abe's Liberal Democratic Party (LDP).

Last month, Okinawa Governor Hirokazu Nakaima approved a landfill project to implement the plan.

Futenma has long been a lightening rod for discontent among Okinawa residents, many of whom associate the concentration of U.S. bases with accidents, pollution and crime such as the 1995 rape of a Japanese schoolgirl by three U.S. servicemen.

Seeking to soothe discontent, Abe's government earmarked 348 billion yen ($3.34 billion)for Okinawa's economic development in the draft budget for the year from April and pledged about 300 billion yen per year through 2021/22.

Abe also promised to study whether the relocation plan could be speeded up and said the government would start talks with the United States on a deal that could allow for more oversight of environmental issues at U.S. bases.

Political analysts said Inamine's win, while likely to slow progress, might not be a death knell for the relocation plan.

"Mr. Inamine says he is totally opposed and may create obstacles, but the Abe government is trying hard to keep its promise to the United States," Seiichi Eto, an LDP lawmaker and aide to Abe, told Reuters late last week.

Analysts say Abe could risk denting voter support for his government, which came to power at the end of 2012 with promises to revive the economy, if he does push ahead with the relocation of the base in the face of local opposition.

"Inamine's victory will give momentum to the anti-base movement and the opposition campaign could spread," Takashi Kawakami, a professor at Takushoku University, said. "Abe will probably try to forge ahead but there will probably be an opposition movement ... and if this is reported in the media daily, Abe's support rates could fall."