The United States successfully carried out a flight test of ground- based missile defense system Friday, shooting down a warhead over the Pacific Ocean and saying it now has a "good chance" to intercept long- range North Korean missiles.

The success, which came after two failed tests in December 2004 and February 2005, is expected to boost the recently stepped-up U.S. efforts to build up its missile shield since North Korea's missile launches on July 5.

"I think we have a good chance" to shoot down long-range North Korean missiles, Missile Defense Agency Director Lt. Gen. Henry Obering said in announcing the successful test involving "an improved ground-based interceptor designed to protect the United States a limited long-range ballistic missile attack."

"And it's one that I feel safer and sleep better at night," Obering told a press conference.

U.S. Defense Secretary Donald Rumsfeld also said in a statement that such successful tests "increase confidence in the approach to developing an initial defense capability."

But he also stressed the need to boost development, saying, "While today's test was a success, the program is by no means complete."

"Tests will continue, some of which will be successful and some will not. This was a challenging test, and the tests will become even more challenging in the period ahead," Rumsfeld said.

While the ground-based system against long-range missiles is still in a test stage, the United States had reportedly activated it earlier this year amid increasing indication of North Korea preparing to test- launch a ballistic missile.

North Korea actually fired seven missiles, including a long-range Taepodong-2 missile, which is believed to have a capacity of reaching Alaska and the West Coast areas in a two-stage version and all of North America with a three-stage variant.

But the United States did not actually have to use the system to intercept the North Korea missile, which failed soon after its launch and tumble into the Sea of Japan.

The ground-based system currently has interceptors missiles deployed at Fort Greely, Alaska, and Vandenberg Air Force Base in California.

In the test Friday, an interceptor was launched from the Vandenberg base against a target missile fired from the Kodiak Launch Complex in Alaska.

"Although not a primary objective for the date collection flight test, an intercept of the target warhead was achieved," said the missile agency of the Defense Department.

The test results "will help to further improve and refine the performance of numerous ground-based midcourse defense elements that will be used to provide a defense against the type of long-range ballistic missile that could be used to attack an American city with a weapon of mass destruction," the agency said.

The elements include an air-transportable X-band radar recently deployed in Japan's Aomori Prefecture. The advanced radar is part of an early warning system for missile launches.

In the previous two failed tests, interceptor missiles did not launch from silos in the Marshall Islands although mock warheads was successfully fired from Kodiak.