In a rebuff to China, US Defense Secretary Ash Carter has said that the United States military would sail and fly wherever international law allowed, including the disputed South China Sea.

Meanwhile, Taiwan today announced it will increase its coast guard presence on a small island in the South China Sea Spratlys, as Beijing asserts its claims to the same chain.

Taiwan has largely kept out of disputes between China and its neighbors in the region, through which $5 trillion in ship-borne trade passes every year. China, the Philippines, Vietnam, Malaysia, Taiwan and Brunei all have overlapping claims.

Rival claims to the island by Taiwan and China go back to before defeated Nationalists fled to Taiwan after losing the Chinese civil war with the Communists in 1949. China to this day considers self-ruled Taiwan a renegade province, to be united with the mainland by force if necessary.

Taiwan's Coast guard chief Wang Chung-yi said construction of a port on the island of Itu Aba, or Taiping as it is known in Taiwan, remains on track and will be able to support permanently stationed 100-ton ships and allow 2,000- and 3,000-ton vessels to dock.

Currently the island supports around 180 people, about 150 of them marine-trained coast guard personnel who have had oversight of the 46-hectare (114-acre) island since 2000.

Carter spoke in Boston after a two-day meeting between US and Australian foreign and defense ministers at which the long-time allies agreed to expand defense cooperation and expressed "strong concerns" over Beijing's building on disputed islands.

"Make no mistake, the United States will fly, sail and operate wherever international law allows, as we do around the world, and the South China Sea will not be an exception," Carter told a joint news conference.

"We will do that in the time and places of our choosing," Carter added.

He had been asked about reports that the US had already decided to conduct freedom-of-navigation operations inside 12-nautical-mile limits that China claims around islands built on reefs in the Spratly archipelago.

In Beijing, when asked about Carter's remarks, Chinese Foreign Ministry spokeswoman Hua Chunying said China had not militarized the South China Sea.

"I want to point out that some countries, in a region far from their own lands, have deployed offensive weaponry on a large scale and flexed their military muscles again and again in the South China Sea," Hua told a daily news briefing.

"This is the biggest factor in the militarization of the South China Sea. We hope the relevant countries cease hyping up the South China Sea issue and scrupulously abide by their promises not to take a position on the territorial disputes."

The Boston meeting brought together Carter, US Secretary of State John Kerry, Australian Foreign Minister Julie Bishop and Australian Defense Minister Marise Payne for regular talks between the two countries.

A joint statement said they "expressed strong concerns over recent Chinese land reclamation and construction activity in the South China Sea. It called on "all claimant states to halt land reclamation, construction, and militarization."

Bishop welcomed a statement by Chinese President Xi Jinping last month that China did not intend to militarize the islands and said she hoped Beijing would stick to the commitment.

China claims most of the South China Sea and last week its foreign ministry warned that Beijing would not stand for violations of its territorial waters in the name of freedom of navigation.

Some analysts in Washington believe the decision has been taken and the patrols could take place later this week or next.

"You know, doing the 12-nautical mile challenge is one among a variety of options that we're considering," a US official said. "We're waiting for an interagency decision that includes the White House."

The US says that under international law building up artificial islands on previously submerged reefs does not entitle a country to claim a territorial limit and that it is vital to maintain freedom of navigation.