"Any irresponsible comments from other countries cannot be accepted," Lieutenant General He Lei said at the Shangri-La Dialogue in Singapore.

The rebuke comes after Australia and the US issued warnings to China about its activities in the South China Sea.

Speaking just hours earlier at the same security summit in Singapore, US Defense Secretary Jim Mattis accused China of intimidation and coercion in the disputed waters.

Also addressing attendees of the summit, Australia's Defence Minister Marise Payne earlier urged China against a "might is right" approach to international relations.

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China defends its actions as 'national defence'

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.

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

The Chinese general, however, said Beijing's actions were aimed at "national defence".

"They are for the purpose of avoiding being invaded by others... As long as it is on your own territory you can deploy the army and you can deploy weapons," he said.

'Might is right' approach is counterproductive: Payne

In her speech to the Shangri-La Dialogue, Senator Payne insisted nations have a right to be free from coercion when they reasonably object to the behaviour of other nations.

Relations with China have soured in the past year, with Beijing especially cranky about Australia's foreign interference laws.

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"When Australia disagrees with the actions of another nation, including partners and allies, we say so," Senator Payne told the summit, which is the largest annual gathering of defence ministers, military chiefs and senior security officials across the region.

She pointed to Australia's stance on the South China Sea, which encourages countries to resolve territorial claims based on international law, as well as the publicly expressed disappointment that the US had withdrawn from the Trans-Pacific Partnership trade pact.

Senator Payne made the case that it was important for the US to continue to play an active role in the region to ensure peace and security.

The speech acknowledged disruption to international relations can create instability but emphasised strategic competition must be bound by rules that shouldn't be casually discarded.

"Adopting a 'might-is-right' approach is contrary to the interests of all nations," she said.

Senator Payne pointed to the recent Australia-East Timor settlement of a long-running maritime boundary dispute using a United Nations conciliation process.

"It's an example of the rules-based order in action," she said.

Growing terrorism threat in southeast Asia

After years of bitter wrangling, Australia and East Timor in March signed a historic treaty at the United Nations to resolve a maritime boundary dispute in March. The deal also carves up $56 billion in potential revenue from oil and gas reserves in the Timor Sea.

Senator Payne also warned of the growing terrorism threat in southeast Asia, highlighting the recent suicide bombings in Surabaya, Indonesia and insurgency in Marawi in the Philippines.

"Nobody wants to see (Islamic State) take root in our region after being denied territory and legitimacy in the Middle East," she said.

Incoming chief of the defence force Angus Campbell is also in Singapore for the dialogue, along with the bosses of the foreign affairs and defence departments.