A US Air Force AWACS (Airborne Warning and Control Systems) aircraft taxis at Kadena US Air Force base in Kadena town on Japan's southern island of Okinawa May 29, 2009. [Agencies]

Support For Hatoyama Could Waver

Some experts in Japan criticised Washington for taking such a tough approach.

"In the past, if the United States came out strong, Japan would fall into line. But that is no longer the case," said Yasunori Sone, a political science professor at Tokyo's Keio University.

"(Japan's) Democratic government is unable to fully read America's strategic intentions and this is appearing as a perception gap," said Takehiko Yamamoto, a professor at Waseda University in Tokyo.

"As this grows, the United States will not be able to view Japan as a reliable partner."

"The prime minister's position is (also) not clear-cut ... This will be one of the major factors that would undermine public support for the cabinet," Yamamoto added.

In a two-day visit to Tokyo that ended on Wednesday, Gates pressed hard for the base deal to go ahead without major changes.

"Our view is clear. The Futenma relocation facility is the lynchpin of the realignment road map," he said on Wednesday.

"Without the Futenma realignment, the Futenma facility, there will be no relocation to Guam. And without relocation to Guam, there will be no consolidation of forces and the return of land in Okinawa."