On October 27, Washington sent the destroyer USS Lassen near the Spratly islands in a move widely believed to be a challenge to China's territorial claims in the region.
McCain, the Republican chair of the Senate Armed Services Committee, said in a November 9 letter to Defense Secretary Ash Carter that it was vital there should be no misunderstanding about US objectives.
"I believe it is critical that the Department of Defense publicly clarify… the legal intent behind this operation and any future operations of a similar nature," McCain wrote in the letter seen by Reuters on Wednesday.
Washington has rejected China's territorial claims in the South China Sea, where Vietnam, Malaysia, Taiwan, Brunei, and the Philippines have overlapping claims.
US officials said last week that because the Navy did not conduct military drills during last month’s patrol, the operation resembled so-called "innocent passage." However, that approach could reinforce rather than challenge China's sovereignty claims, according to experts.
Innocent passage occurs when a ship quickly passes through another country's territorial waters, and it can only take place in waters belonging to another country.
McCain called on Carter to clarify what excessive claims the Lassen was intending to challenge and whether the warship operated under the rules of innocent passage, Reuters reported.
Pentagon officials have given conflicting descriptions of the Lassen's maneuver.
A US official speaking to Reuters at the time described it as an "innocent-passage" operation, but later said that had been a mistake.
Pentagon spokesman Captain Jeff Davis said the patrol was not an "innocent passage," but when pressed further the following day, he declined to explicitly restate that position or elaborate.
The Pentagon has yet to respond to McCain's letter, Reuters reported.
