WASHINGTON — The commander of American military forces in the Pacific region said Thursday that warships equipped with advanced radar and other ballistic-missile defense systems were being relocated to monitor a potential rocket launching by North Korea.

The commander, Adm. Samuel J. Locklear III, said the American military was watching for the expected North Korean operation “very closely,” and he reiterated that the launching of any long-range missile would violate United Nations Security Council resolutions.

North Korea announced Saturday that it would try to launch another long-range rocket before the end of the year. The launching, expected sometime between Monday and Dec. 22, would commemorate the death a year ago of North Korea’s longtime ruler, Kim Jong-il, and would come as his son Kim Jong-un, the current leader, tries to build credibility.

During a Pentagon news conference on Thursday, Admiral Locklear said the goal of moving the ships would be to track a missile launching to determine “what kind is it? What is it about? Where does it go? Who does it threaten?”