Beijing said last week two American warships sailed near disputed islands in the South China Sea without permission, prompting the Chinese Navy to ask them to leave.

The ships entered waters adjacent to Gaven and Chigua reefs in the Spratly Islands, which Beijing calls Nansha, on May 6, China's foreign ministry said.

Speaking on the sidelines of a maritime security conference in Singapore, Admiral John Richardson said Washington will continue such operations which are aimed at ensuring freedom of navigation.

The US will however ensure that communications with Beijing remain open to prevent any untoward incidents, he added.

"I really value the channel of communication that I have with Shen Jinlong," he told reporters, referring to his Chinese counterpart.

"We just recently visited China, we had a chance to get to know each other, understand each other more thoroughly.

"We can continue to advocate that while we may not see things the same in all parts of the world, we've got to work through those differences in a way that doesn't boil over into conflict."

The US Navy regularly conducts freedom of navigation operations to challenge Beijing's vast claims in the sea, often angering China.

After last week's sail-by, a foreign ministry spokesman said "the Chinese side expresses strong dissatisfaction and resolute opposition" to the US action.

But Richardson insisted the patrols were routine.

"We haven't done anything increasingly provocative or anything else that we would not do anywhere else in the world," he said.

Beijing has built artificial islands and military installations in the South China Sea, including on the Spratlys.

China claims nearly all of the sea, but Taiwan, the Philippines, Brunei, Malaysia and Vietnam all claim parts of it.