China has denounced the Japanese PM and US defence secretary for making "provocative" speeches against China at an Asian security forum in Singapore.
Chinese army general Wang Guanzhong said Chuck Hagel and Shinzo Abe's comments at the Shangri-La Dialogue were "unacceptable".
Hagel had earlier said China was "destabilising" the South China Sea.
Meanwhile, Japan's PM Shinzo Abe had vowed to give greater support to South-East Asian countries.
The forum, which brings together the US and South-East Asian countries, comes amid growing tensions between China, Vietnam and the Philippines, with Japan-China ties also strained over disputed islands in the East China Sea.
He said it was "unimaginable" to receive such "unwanted criticisms against China".
In a keynote address on Friday, Japan's Shinzo Abe outlined his vision for a more robust role in resolving territorial disputes in the region.
He also offered to provide coastal boats to neighbouring countries wary of Beijing's tactics.
Chinese officials responded at the time by saying Abe was using the "myth" of a China threat to strengthen Japan's security policy.
Chuck Hagel later weighed in, accusing China of threatening the region's long-term progress by undertaking "destabilising, unilateral actions asserting its claims in the South China Sea".
He warned the US would "not look the other way" when nations ignored international rules.
Tensions have flared recently, with China declaring an air defence zone in the East China Sea and adopting a more confrontational stance over the disputed islands in the South China Sea, correspondents say.
They say that although some Asean members will be reluctant to antagonise China because of their economic and political ties, others are likely to welcome an increased role from Japan.
Beijing claims a U-shaped swathe of the South China Sea that covers areas other South-East Asian nations say are their territory.