BOSTON — Defense Secretary Ashton Carter said Tuesday that U.S. ships and aircraft would fly wherever international law permits despite China's territorial claims in the disputed South China Sea.

Carter made the remarks after signing a defense cooperation statement with Australia amid growing concerns over China's military expansion.

"Make no mistake, we will fly, sail and operate wherever international law permits," Carter said in response to a question about China's claims over islands it has built in the South China Sea. "We will do that in the time and place of our choosing."

Carter's comments come as the United States considers conducting naval patrols near the disputed Chinese-built islands.

The Pentagon says it regularly conducts "freedom of navigation operations" around the world to challenge what the U.S. considers excessive maritime claims. Last year the claims of 18 nations were challenged by such patrols, the Pentagon said.

Carter said the South China Sea would not be an exception.

The United States has not yet challenged the claims surrounding the Chinese-built islands by conducting patrols within 12 miles of the islands. Under international law, territorial waters extend 12 miles from the coast.

The United States says the artificial islands, which include airstrips and aircraft hangers, represent a militarization of the area and are a violation of international laws guaranteeing freedom of navigation. China has denied the islands serve a military purpose.

Carter and Secretary of State John Kerry met with their Australian counterparts, Defense Minister Marise Payne and Foreign Minister Julie Bishop, on Monday and Tuesday in Boston to discuss expanded defense cooperation between the two countries.

The cooperation would include enhanced joint training and working to build military capacities among allies in the region.

Australia has contributed troops in Afghanistan and has been part of the U.S.-led coalition fighting Islamic State militants in Iraq, where Australia has 900 troops, second only to the United States.