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US President Barack Obama has urged China to abide by its obligations under an international treaty in its activities in the South China Sea.

He made the comments during a "candid exchange" with his Chinese counterpart, Xi Jinping, in Hangzhou before the G20 summit, the White House said.

In July, an international tribunal ruled against Chinese claims to rights in the South China Sea.

China dismissed the ruling and said it would not be bound by it.

The ruling was made by an arbitration tribunal under the , which both China and the country that brought the case - the Philippines - have signed.

The White House said Mr Obama emphasised "the importance for China, as a signatory to UNCLOS, to abide by its obligations under that treaty, which the United States views as critical to maintaining the rules-based international order".

Chinese media reports on the meeting between Mr Obama and Mr Xi made no mention of any discussion about the South China Sea.

China claims almost all of the South China Sea, including reefs and islands also claimed by other nations, and has caused dismay in the region by building artificial islands and restricting access.

Earlier on Saturday, the US and China - together responsible for 40% of the world's carbon emissions - both formally joined the Paris global climate agreement.

"History will judge today's effort as pivotal," Mr Obama said.

Last December, countries agreed to cut emissions in an attempt to keep the global average rise in temperatures below 2C.