The Pentagon said Wednesday the heavy bombers had taken part in a combined operation with Japan over the East China Sea and had flown through international airspace over the South China Sea a day before.

"Regarding the provocative actions of US military aircraft in the South China Sea, we are always resolutely opposed to them, and will continue to take necessary measures in order to strongly handle (this issue)," Chinese defence ministry spokesman Ren Guoqiang told a monthly news briefing.

China has claimed large swaths of the strategic waterway and built up a series of islands and maritime features, turning them into military facilities.

Brunei, Malaysia, the Philippines, Taiwan and Vietnam have competing claims to the region, and an international maritime tribunal ruled in 2016 that China's claims have no legal basis.

Pentagon spokesman Lieutenant Colonel Dave Eastburn said this week's flights were part of "regularly scheduled operations."

The United States rejects China's territorial claims and routinely says the military will "continue to fly, sail and operate wherever international law allows at times and places of our choosing."

Washington this week enacted new tariffs against China covering another $200 billion of its imports while it last week sanctioned a Chinese military organisation for buying Russian weapons.

China has reacted angrily, and this week scrapped a US warship's planned port visit to Hong Kong and cancelled a meeting between the head of the Chinese navy and his American counterpart.

US Defense Secretary Jim Mattis said he was not concerned the US transit flights would increase tensions with China.

"If it was 20 years ago and they have not militarised those features there, it would have just been another bomber on its way to Diego Garcia or whatever," he told Pentagon reporters, referring to the US military base in the Indian Ocean.

"So there's nothing out of the ordinary about it, nor about our ships sailing through there."

The Pentagon chief went on to say there is no "fundamental shift in anything."

"We're just going through one of those periodic points where we've got to learn to manage our differences," he said.