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, U.S. 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."
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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 U.S. official said. "We're waiting for an inter-agency decision that includes the White House."
The United States 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 in a sea through which more than $5 trillion of world trade passes every year.