China and other nations must strengthen efforts to curb North Korea's nuclear weapons programme, US Secretary of State Rex Tillerson said Monday, while also calling out Beijing over its South China Sea activities.

America's top diplomat, speaking after talks in Sydney, also gave a brief response to the unfolding crisis in the Gulf, where Saudi Arabia, Bahrain, the United Arab Emirates and Egypt have all announced they are severing ties with gas-rich Qatar.

In the Asia-Pacific region, the United States has spent recent weeks trying to reassure allies it can maintain a tough line against China's "militarisation" of the South China Sea while at the same time seeking help from Beijing.

President Donald Trump -- who frequently denounced China on the campaign trail -- has turned to Beijing to help rein in ally North Korea's weapons programme, prompting concern among Asian allies that America might go easy on the South China Sea territorial dispute.

"We desire productive relationships," Tillerson said after annual discussions with his Australian counterpart Julie Bishop in Sydney.

"But we cannot allow China to use its economic power to buy its way out of other problems, whether it's militarising islands in the South China Sea or failing to put appropriate pressure on North Korea."

He said China and other regional partners should "step up" efforts to help solve the North Korea situation, because it presents a threat to the "entire world."

China claims nearly all of the South China Sea despite partial counter-claims from Taiwan, the Philippines, Brunei, Malaysia and Vietnam.

It has rapidly built reefs into artificial islands capable of hosting military planes.

Tillerson reiterated US and Australian commitment to freedom of navigation in the South China Sea to "ensure unimpeded flow of lawful commerce in a rules-based order".

But reporters asked Tillerson if America was applying a double standard in telling countries to adhere to the international order while simultaneously pulling out of a trans-Pacific trade deal and the Paris climate accords -- moves that prompted even longstanding allies to question whether America was retreating into isolationism.

"That's why we're here, that's why we travel to the region, that’s why we engage with our counterparts," Tillerson said, standing alongside Pentagon chief Jim Mattis, Bishop and Australian Defence Minister Marise Payne.

We "travel to the region to meet with our counterparts and talk about all the issues that are important to them and hear from them concerns about where the (Trump) administration is positioned".

- 'Remain united' -

Addressing the situation in the Gulf, Tillerson called on countries there to stay united and work out their differences.

"We certainly would encourage the parties to sit down together and address these differences," he said.

"If there's any role that we can play in terms of helping them address those, we think it is important that the GCC (Gulf Cooperation Council) remain united."

Riyadh cut diplomatic relations and closed borders with its neighbour Qatar to "protect its national security from the dangers of terrorism and extremism", the official Saudi Press Agency said.

Tillerson and Mattis both said they did not anticipate any impact on efforts by a US-led coalition to battle the Islamic State group in Iraq and Syria. 

The coalition currently conducts much of its operational planning and coordination from Al-Udeid air base in Qatar.

"I am confident there will be no implications coming out of this diplomatic situation at all, and I say that based on the commitment that each of these nations... have made to this fight," Mattis said. 

The US defense secretary  blasted Iran for its "various destabilising efforts" in the region, referring to Iranian support for Syrian President Bashar al-Assad and Tehran's involvement in the Yemen war.

Source: AFP