The US will continue naval patrols in the disputed South China Sea, Washington's National Security Adviser Susan Rice told Chinese representatives during a series of meetings in Beijing, a senior American official said Tuesday.

Rice is among the highest-level US officials to visit China since an international tribunal this month rejected its vast territorial claims in the strategically vital region -- infuriating Beijing and fuelling tensions with Washington.

Her trip was intended to prepare for a visit by President Barack Obama to a G20 summit in Hangzhou in September.

But the question of how to deal with the festering issue, in which Washington has played a prominent role, cast a long shadow over the talks, which included a meeting with President Xi Jinping.

In recent months Washington has sent naval vessels close to reefs and outcrops claimed by Beijing to assert the principle of freedom of navigation, sparking anger in China which has built a series of artificial islands in the area capable of supporting military operations.

In her meetings with top diplomatic and military officials, Rice told her counterparts that "those operations are lawful. They will continue", according to a senior US official, who asked for anonymity to discuss the sensitive subject.

The issue was not directly raised with Xi, he said, describing the conversation as "incredibly positive", although "there was a very clear recognition that we face a number of challenges".

In general terms, he said, "both sides were very clear with one another".

"There's no room for ambiguity," he added. "That kind of clarity... promotes stability and reduces the risk of miscalculation."

- 'Risk of miscalculation' -

A tribunal at the Permanent Court of Arbitration in The Hague on July 12 denied the legal basis for Beijing's claim to nearly all of the sea, parts of which are also claimed by neighbouring nations.

Beijing rejected the ruling as "waste paper" and asserted its right to declare an Air Defence Identification Zone controlling flights over the area.

Rice "stressed the importance of all parties taking steps to reduce tensions. To avoid taking actions that... could raise the risk of miscalculation", the official said.

Instead, Rice called on Beijing to use the ruling as an opportunity to "reinvigorate diplomacy" in the region, he added.

In remarks before the meetings Monday, Fan Changlong, vice chairman of China's Central Military Commission warned that ties between the two powers could easily fray.

"We should be honest with ourselves that deep down in this relationship we're still faced with obstacles and challenges," he said, adding that military ties had been "impacted by some complicated and some sensitive factors".

"If we do not properly handle these factors it will very likely disturb and undermine this steady momentum of our military-to-military relationship," he warned.

Beijing objects to an agreement by Washington and Seoul to deploy a US missile defence system to South Korea.

Rice told Fan the move was "purely a defensive measure" and "not aimed at any other party other than North Korea and the threat it poses," the official said.