Prime Minister Yoshihiko Noda told his navy on Sunday that Japan's security environment was tougher than ever, underscoring tension with China over a territorial dispute and the threat of North Korea's weapons programmes.
Sino-Japanese relations deteriorated sharply after Japan in September bought from private owners some of the East China Sea islets that both Tokyo and Beijing claim. That sparked violent anti-Japanese protests across China and badly hurt trade.
"It is needless to say that the security environment surrounding Japan is getting tougher than ever," Noda told about 8,000 servicemen and women, mostly from the navy, from aboard the destroyer Kurama.
"We have a neighbour that launches missiles under the pretence of satellite launches. We have various developments concerning territory and sovereignty."
Noda, supreme commander of Japan's military known as the Self-Defence Forces (SDF), was speaking during a fleet review off Kanagawa prefecture, south of Tokyo.
His remarks were relayed to ships gathered in the area -- about 40 vessels, including the U.S. cruiser Shiloh, were present for the review. About 47,000 U.S. troops are based in Japan.
Noda braved occasional bouts of drizzle to review destroyers, submarines, minesweepers and fuel supply vessels that passed in front of him while SDF helicopters and P-3C anti-submarine patrol planes flew overhead.
The disputed group of islands, called Senkaku in Japan and Diaoyu in China, are located near rich fishing grounds and potentially huge oil and gas reserves.
Patrol vessels from Japan and China have been keeping a wary watch on each other in the East China Sea near the islets, raising concerns that an unintended collision or other incident could develop into a broader clash.
Japan's Coast Guard had planned to send one of its patrol ships to the naval review, but it cancelled the vessel's participation because the heightened tensions prompted it to allocate more ships to the area.
Japan is also wary of North Korea's nuclear and missile projects, with its territory lying within the range of Pyongyang's missiles.
North Korea has conducted three rocket tests since 2006 for what Pyongyang says are satellite launches, though Japan and Western countries suspect they are intended to test missiles. The North has also conducted two nuclear tests.