Senator John McCain said in a statement that the lack of US action was allowing China to continue to "pursue its territorial ambitions" in the region, most recently by landing a plane on a man-made island in the Spratly Islands archipelago on Saturday.
McCain said the lack of additional US patrols last year was "disappointing yet hardly surprising." He said the Obama administration was "either unable to manage the complexities of interagency national security decision-making or simply too risk averse to do what is necessary to safeguard the rules-based order in the Asia-Pacific."
Analysts say China's increasing military presence in the disputed sea could ultimately lead to a Beijing-controlled air defence zone, ratcheting up tensions with other claimants and the United States.
US State Department spokesman John Kirby on Monday said China's first landing of a plane on an island in the disputed region "raises tensions and threatens regional stability."
"We again call for all claimants to halt land reclamation and further development of new facilities and militarization on their outposts and instead focus on reaching agreement on acceptable behavior in disputed areas," he told reporters.
US officials remain committed to carrying out further "freedom of navigation" patrols near the dispute islands, but are still debating the timing of another patrol, said one U.S. defense official, who was not authorized to speak publicly.
"The question is do we want to escalate the situation and ratchet it up?" said the official.
US Defense Secretary Ash Carter told McCain in a letter dated December 21 that the Navy conducted a previous patrol in October to be "lawful under all possible scenarios" given ambiguities about whether certain islands in the region are entitled to a territorial sea. He said the United States would continue to "fly, sail and operate wherever international law allows."
Carter said the October 27 patrol included a continuous transit consistent with what is known as the "right of innocent passage," which applies only in a territorial sea, and with "freedom of navigation," which applies beyond those limits.
At the time, critics said the destroyer Lassen's decision to conduct an "innocent passage" by skipping military drills could have actually reinforced China's claim to sovereignty over the islands. Such passage can only take place in waters belonging to another country.
Carter said the patrol was not meant to challenge any claims of sovereignty, but to challenge attempts by the countries involved to restrict navigation rights and to require prior notification of transits.
He said US officials did not notify any of the countries prior to the transit patrol.
Washington argues that islands China has built up in the South China Sea are not entitled to a territorial limit under international law as they used to be underwater at high tide.
China argues that the islands would be used mainly for civilian use, such as coast guard activity and fishing research.