Such moves could involve longer patrols, ones involving larger numbers of ships or operations involving closer surveillance of Chinese facilities in the area, which now include electronic jamming equipment and advanced military radars.
US officials are also pushing international allies and partners to increase their own naval deployments through the vital trade route as China strengthens its military capabilities on both the Paracel and Spratly islands, the diplomats said, even if they stopped short of directly challenging Chinese holdings.
"What we have seen in the last few weeks is just the start, significantly more is being planned," said one Western diplomat, referring to a freedom of navigation patrol late last month that used two US ships for the first time.
"There is a real sense more needs to be done."
The Pentagon does not comment on future operations but a spokesman, Lieutenant Colonel Christopher Logan, said "we will continue to work with our friends, partners, and allies to ensure a free and open Indo-Pacific".
A more assertive Pentagon approach already appears to have started. Reuters first reported the patrol last month in which two US Navy warships sailed near South China Sea islands claimed by China, even as President Donald Trump sought Chinese co-operation on North Korea.
US Defence Secretary Jim Mattis warned in Singapore on Saturday that China's militarisation of the South China Sea was now a "reality" but that Beijing would face unspecified consequences.