US welcomes start of base move on Japan's Okinawa
The United States said it welcomes the start of Japan's relocation of a US military base on the island of Okinawa.
Japan has placed buoys in a bay in the north of the island to cordon off an area earmarked for the new base.
The two sides agreed in 1996 on the move, which involves relocating Futenma airbase from a highly-congested part of the island to Nago in the north.
But residents have long opposed the new base, saying it will damage marine life. They want it moved off Okinawa.
Okinawa is home to several US bases and around 26,000 US troops. Many residents associate the US bases with noise, accidents and crime.
The US State Department spokesman Marie Harf told reporters on Thursday that the progress being made was "a critical step" towards the US and Japan's plan for the realignment of US forces on Okinawa.
She said the relocation of the Marine Corps airbase would reduce the US footprint in the most populated part of Okinawa.
It would also "enable the return of significant land back to the people of Okinawa while sustaining US military capabilities vital to the peace and security of the region".
About 200 protestors rallied near the site of the new base off Camp Schwab at Henoko in Nago city on Thursday, reports said. Some took to boats to protest and coastguard ships were deployed.
The mayor of Nago said in a statement that he was "infuriated" by the start of work.
"We strongly protest this outrageous move by the Japanese government and are determined to block any construction of a new base," said Susumu Inamine, who was re-elected in January.
Mr Inamine has previously threatened to reject procedures linked to the base's relocation, including denying permits for the project.
In December 2013, the Okinawa governor finally approved a landfill that will enable construction of the new airbase - which will be located in Oura Bay off an existing base called Camp Schwab - to start.
The US bases on the island form a part of its long-standing security alliance with Japan. There has been a US military presence on Okinawa since the end of World War II.
But resentment at the US presence has been growing among Okinawans, particularly since the 1995 gang-rape of a 12-year-old girl by US troops.