BEIJING - China is "disappointed to the point of despair" with Japan's conduct in the South China Sea, it said Monday, after Tokyo announced it may set up training patrols with the US in the contested region.

China asserts sovereignty over almost all of the strategically vital waters in the face of rival claims from its Southeast Asian neighbours, and has rapidly turned reefs in the area into artificial islands capable of hosting military planes.

In a speech last week Japanese defence minister Tomomi Inada called China's actions a "deliberate attempt to unilaterally change the status quo, achieve a fait accompli, and undermine the prevailing norms", according to a transcript released by Washington-based think tank the Center for Strategic and International Studies.

Inada said Japan would increase its engagement in the South China Sea through joint training cruises with the US Navy, exercises with regional navies and capacity-building assistance to coastal nations.

Japan's conduct in the South China Sea "makes one feel disappointed to the point of despair," foreign ministry spokesman Lu Kang said at a regular press briefing Monday, before accusing Japan of meddling in a situation that ought to be resolved via "direct negotiation between involved parties".

Japan was attempting to confuse the situation in the region Lu said, adding the island nation had "even resorted to deception in attempts to impose its own views" on the issue on other countries.

Japan has a separate territorial dispute with China over uninhabited islets in the East China Sea.

Last week, China sailed four coastguard vessels into that contested region, sparking complaints from Japan that the Asian giant was escalating tensions.

Japan is a key US ally, and is boosting defence ties with the Philippines and other Southeast Asian nations that have their own territorial disputes with Beijing over the South China Sea.

In recent months, Japanese Prime Minister Shinzo Abe has criticised China for rejecting a July ruling by an international tribunal that found Beijing's extensive claims to the waters had no legal basis.

But Lu warned: "China is unwavering in its determination to safeguard its territorial sovereignty and maritime rights and interests."