The United States and its allies pushed Thursday for accelerated efforts to lower tension in the disputed South China Sea, where Beijing's assertive sovereignty claims have raised fears of potential conflict.

A day after China showed a commitment to an eventual code of conduct to prevent conflict in the strategic body of water, Japan and the Philippines joined Washington's calls to speed-up diplomacy.

US Secretary of State John Kerry told reporters on the sidelines of an East Asia Summit in Brunei that such a code was needed.

"A code of conduct is a necessity for the long term, but nations can also reduce the risk of miscommunication and miscalculation by taking steps today," Kerry said, according to a copy of his remarks to the summit.

Kerry did not single out any countries by name, but China has come under growing pressure over its claims to virtually all of the body of water, and acts interpreted by some of its neighbours as aggressive.

Kerry added "all claimants have a responsibility to clarify and align their claims with international law".

Prime Minister Shinzo Abe of Japan, which itself is embroiled in a heated dispute with China over islands and waters between the two powers, told reporters he "looked forward to the early conclusion of a code of conduct which is legally binding.

"The sea should be ruled by law and not by force," he added.

Philippine President Benigno Aquino told the closed-door summit a code of conduct should be completed "as soon as possible," according to a statement by his government.

Agreement on a code of conduct has long been a stumbling block between China and some Southeast Asian nations, particularly rival claimants the Philippines and Vietnam.

Malaysia and summit hosts Brunei also have competing claims to parts of the sea.

China has struck a friendlier tone in recent months, agreeing to discuss the code with the 10-nation Association of Southeast Asian Nations (ASEAN).

The move is seen by some analysts as little more than a stalling tactic.

A promise to discuss the code delivered to ASEAN leaders on Wednesday by China's Premier Li Keqiang "means that China will use consensus building to slow the consultation process down", Carl Thayer, a China specialist at of the Australian Defence Force Academy told AFP.

The South China Sea is believed to hold vast deposits of oil and natural gas and is regarded as a potential military flashpoint.