U.S. Navy officials confirmed that the USS Curtis Wilbur, one of three ships in the U.S. 7th Fleet tasked with the patrols, left its base just south of Tokyo earlier this week.
They refused to comment on details of the destroyer's mission for security reasons. But Navy Secretary Gordon England said in March that the patrols would begin Oct. 1, and fleet officials confirmed that there has been no change in the schedule.
The other two destroyers assigned to the mission remain in port at Yokosuka Naval Base, the fleet's home.
The patrols are an initial step toward fulfilling a promise President Bush made two years ago to erect a ballistic missile shield that would protect the United States, its allies and its troops abroad from attack.
Bush cleared the way for the system by withdrawing from the 1972 Anti-Ballistic Missile Treaty, which banned ship-based missile defenses. He's called the plan one of his administration's top priorities.
Critics, however, say such a shield would be too complex to be effective. It's estimated to cost $51 billion over the next five years.
Starting the program in the Pacific underscores Washington's increasing concern over North Korea's suspected development of nuclear weapons and long-range missiles capable of reaching Alaska or perhaps even western mainland states.
All of Japan is already within reach of the North's missiles, as are the more than 50,000 U.S. troops deployed here, including the 20,000-strong 7th Fleet, the Navy's largest and the only one with a home port outside of the United States.
North Korea's state-run media was quick to denounce the deployment.
"The U.S. should clearly understand that a preemptive attack is not its monopoly," North Korea's Rodong Sinmun said in a commentary Friday, adding that the deployment of the destroyer "proves that the U.S. attempt to invade the DPRK has reached a serious phase of implementation."
DPRK is an abbreviation of North Korea's official name.
The North shocked Japan in 1998 by launching a multistage "Taepodong" ballistic missile over Japan's main island. Pyongyang agreed in 2002 to a moratorium on further long-range tests, but reports of increased activity at North Korean missile sites last week led to fears that it may soon carry out a missile test.
Under the U.S. plan, the 7th Fleet destroyers will carry out long-range searches and tracking of missile activity.
Eventually, data gleaned by the ships would be transmitted to Ft. Greely in Alaska and Vandenberg Air Force Base in California, where, if necessary, interceptor missiles would be launched. The interceptors won't be fully deployed at the American bases until next year.
Senior Navy officials have said the United States intends to maintain a virtually continuous presence in the Sea of Japan, which separates Japan from the Korean Peninsula, China and Russia.
The three ships in the 7th Fleet assigned to carry out the patrols are the USS Fitzgerald, the USS Curtis Wilbur and the USS John S. McCain.