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 added "all claimants have a responsibility to clarify and align their claims with international law".
More From This Section
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.
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.