China and Russia blasted Washington Tuesday for its decision to deploy a missile defence system in South Korea in response to Pyongyang's nuclear programme.

Speaking at the 7th annual Xiangshan regional defence forum in Beijing, top military officials from the countries said the move by Seoul and Washington represented a threat to regional stability and was a step towards a new global arms race.

The US and South Korea agreed to install the system, known as THAAD, this summer following repeated nuclear and missile tests by Beijing's wayward ally Pyongyang.

Washington was using the North's actions as a pretext to gain military superiority over China, Chinese general Cai Jun told defence officials at a briefing on the forum's sidelines.

Deploying the system in South Korea, he said, was "not conducive to the peace and stability of the Korean peninsula", he said, adding "it has increased the risk of military conflict in the region".

Beijing fears that THAAD could be used against its own missiles, effectively undermining its nuclear deterrence capabilities against the US.

From a global perspective, Cai said, the US's insistence on developing its missile defence programme could trigger "an arms race at a high level, even to outer space."

Speaking on a panel during the forum, Russian deputy defence minister Anatoly Antonov also slammed the agreement.

"We are concerned about the attempts of certain nations to exploit the complex situation in the Korean peninsula," he said, "pumping this sub-region with clearly excessive defence capabilities."

The decision has "aggravated regional tension" and "adds problems to solving the situation."

North Korea conducted its fifth nuclear test last month, drawing international censure and a push to put further sanctions on the isolated country.

- Flashpoint for conflict -

Tuesday's meeting -- themed on "building a new type of international relations" -- followed a year of growing tension in the region, primarily centred on Beijing's actions in the strategically vital South China Sea, where it has built islands capable of supporting military facilities.

China's defence minister Chang Wanquan opened the forum with a thinly veiled criticism of recent US involvement in Asian trouble spots.

"Some countries seek absolute military superiority, ceaselessly strengthen their military alliances, and seek their own absolute security at the costs of other countries' security," Chang told defence officials and academics at the meeting.

The US under President Barack Obama has sought to "pivot" to Asia by increasing military and economic engagement in the region, raising concerns in Beijing that Washington is working to contain the Asian giant's growing power.

Beijing says it has rights to almost the entire South China Sea, while Brunei, Malaysia, the Philippines and Vietnam, and Taiwan all have rival claims.

In July, an international tribunal said that almost all of Beijing's claims have no legal basis, a decision China attacked as biased and vowed to ignore.

Washington argues that Beijing's moves pose a challenge to freedom of navigation and has sought to challenge Beijing's claims by dispatching military ships and aircraft to the area.

Addressing the forum, former Australian Prime Minister Robert Hawke warned that the disputes, if not properly managed, could become "a flashpoint for US-China conflict".

"These disputes have the potential to disrupt the entire regional order," he said.