Image copyright AP Image caption The USS Lassen, seen here in a file photo, entered an area claimed by China in the South China Sea last week

The US has said it plans to return to contested areas of the South China Sea, with a top military commander saying it has conducted similar operations worldwide "for decades".

The sailing of a US warship within an area claimed by China last week angered Beijing, which issued a warning.

Adm Harry Harris, speaking in Beijing, said such moves should surprise no-one.

An unnamed US official earlier told Reuters similar patrols would take place at least "twice a quarter".

The guided-missile destroyer USS Lassen breached the 12-nautical mile zone that China claims around the Subi and Mischief reefs in the Spratly archipelago.

Contested South China Sea

Image copyright Reuters Image caption China has been steadily reclaiming land around reefs in the South China Sea

Admiral Harris, of the US Pacific Command, said in a speech at Peking University: "We've been conducting freedom of navigation operations all over the world for decades, so no-one should be surprised by them."

"Our military will continue to fly, sail, and operate whenever and wherever international law allows. The South China Sea is not - and will not - be an exception."

But he also added that the operations, which he described as "routine", should "never be construed as a threat to any nation".

'Expect more'

His remarks came a day after US deputy national security adviser Ben Rhodes said ensuring freedom of navigation in the South China Sea was in the US interest.

Defense One, which organised the event he was speaking at, quoted him telling the audience to "expect more demonstrations of that interest".

"We have a responsibility to demonstrate that we're going to be there but fostering peaceful resolutions," he added.

Separately, an unnamed US official told Reuters that military visits to the region could take place as often as "twice a quarter or a little more than that".

"That's the right amount to make it regular but not a constant poke in the eye. It meets the intent to regularly exercise our rights under international law and remind the Chinese and others about our view," the official said.

The US and China's naval commanders spoke by video link last week after the USS Lassen's trip. China told the US that a minor incident could spark conflict in South China Sea if the US did not stop its "provocative acts".

Tensions have escalated in the resource-rich South China Sea in recent years, where several countries have overlapping maritime claims, as China has steadily expanded and consolidated its presence.

China, which claims a wide swathe of the sea, has been reclaiming land around reefs and constructing airstrips and buildings. The US and other countries have called for the halt of such activities, accusing it of militarisation, but China has insisted that the construction is for civilian purposes.