It was "logical" that US naval ships in the region would be employed to track North Korea's launch and "to the degree that those those ships are capable of participating in ballistic missile defense, then we'll position them to be able to do that," said Admiral Samuel Locklear of the US Pacific Command.
American naval ships had been sent to the region "so we understand if they do violate the UN Security Council (resolution) and launch a missile, what kind is it? What is it about? Where does it go? Who's threatened?" he said.
The admiral added that US forces would also be tracking any stray parts that might fall from the rocket.
Japan's also deployed three destroyers armed with missile interceptors to waters over which the rocket might pass.
The three Japanese Aegis destroyers are bound for the East China Sea and the Sea of Japan.
The captains of the destroyers have been ordered to shoot down the North Korean rocket if it looks like falling onto Japanese territory.
Pyongyang has announced it will conduct between December 10 and 22 its second long-range rocket launch this year, after a much-hyped but failed attempt in April.
The North says it will be a purely "peaceful, scientific" mission aimed at placing an Earth observation satellite into orbit.
The United States and its allies insist the launches are disguised tests for an inter-continental ballistic missile capable of carrying a nuclear warhead.
Admiral Locklear said North Korea has steadily improved its missile technology but it was unclear if the test this month would be successful.
"I think they have progressively gained better technology over time through a number of methods, a number of years and decades.
"To the degree that they would be more successful than they were last time in such a short period of time and what they've done to correct it, I can't tell you."