China's military build-up in the South China Sea and its deployment of high-end weapons systems in the disputed waters is designed to intimidate and coerce neighbours, US Defense Secretary Jim Mattis said Saturday.

His broadside at a security summit in Singapore drew a sharp rebuke from a Chinese general, who lashed out at "irresponsible comments" on the contested sea and insisted Bejing was simply defending its territory.

Speaking just 10 days before President Donald Trump is due to meet North Korean leader Kim Jong Un, Mattis also said the US military continues to support diplomats pushing for the "complete, verifiable and irreversible" denuclearisation of the Korean peninsula.

Beijing has deployed a range of military hardware including anti-ship missiles, surface-to-air missiles and electronic jammers across the South China Sea, where it has built islets and other maritime features into hardened military facilities, Mattis said.

China has also landed heavy bombers on Woody Island in the Paracel Islands.

"Despite China's claims to the contrary, the placement of these weapon systems is tied directly to military use for the purposes of intimidation and coercion," Mattis told the Shangri-La Dialogue.

He also blasted Chinese President Xi Jinping for reneging on a 2015 promise made at the White House that Beijing would not militarise the island features in the South China Sea.