WASHINGTON — The Pentagon successfully tested a U.S. 
long-range interceptor missile over the Pacific Ocean on 
Tuesday in an exercise aimed at helping gauge American
 readiness to counter a potential threat from North Korea.
During the test, the Pentagon's Missile Defense Agency
 launched an interceptor rocket from an underground silo
 at Vandenberg Air Force Base in California. The interceptor
 hit and destroyed an intercontinental-range missile fired 
from a test site on Kwajalein Atoll in the Pacific, the 
Pentagon announced.
U.S. Missile Defense Agency Director Vice Adm. Jim 
Syring called the test a "critical milestone" in a statement.
"This system is vitally important to the defense of our
 homeland, and this test demonstrates that we have a
 capable, credible deterrent against a very real threat.
 I am incredibly proud of the warfighters who executed
 this test and who operate this system every day," Syring said.
 US Fires Missile to Test System Designed to Protect 
US from Missile Attack 1:47
The U.S. military's test comes on the heels of North Korea's
 landed in Japan's maritime economic zone. It also comes
 amid heightened tensions between the U.S. and Pyongyang
 over North Korea's continued provocations under leader 
Kim Jong Un.
North Korea has said it is working on an ICBM, which could
 potentially hit the West Coast, and American officials are
 concerned about the possibility the North Koreans could
 miniaturize a warhead to put on an ICBM.
General John Hyten, the head of U.S. Strategic Command, 
has said Pyongyang already has ICBM capability, but still
 lacks a miniaturized warhead.
The $244 million test does not necessarily confirm that the 
American military is capable of defending itself against an
 intercontinental-range missile fired by North Korea.
 Pyongyang also is understood to be moving closer to the
 capability of putting a nuclear warhead on such a missile 
and could have developed decoys sophisticated enough to
 trick an interceptor into missing the real warhead.
President Donald Trump was briefed after a North Korean
 missile test earlier this month and the White House said
 in a statement at the time that "North Korea has been
 a flagrant menace for far too long."
During a recent visit to South Korea, Secretary of State
 Rex Tillerson said "the policy of strategic patience (with 
North Korea) has ended," and he added that military 
action could be on the table if North Korea elevates the
 threat of its weapons programs.
Before Tuesday, the most recent intercept test, in June
 2014, also was successful, but the longer track record
 is spotty. Since the system was declared ready for 
potential combat use in 2004, only four of nine intercept
 attempts have been successful.
Image: A Ground-based Midcourse Defense interceptor is launched from the Vandenberg Air Force Base, California on June 22, 2014.
A Ground-based Midcourse Defense interceptor is launched from the Vandenberg Air Force Base, California on June 22, 2014. Michael Peterson / MDA
and missile programs are a defense against perceived U.S. military threats.
Laura Grego, 
senior scientist 
at the Union of 
Concerned 
Scientists, which
 has criticized the
 U.S. missile 
defense program, 
calls the interceptor
 an "advanced 
prototype," 
meaning it is not fully matured technologically even if it has 
been deployed and theoretically available for combat since 
2004.
The interceptors are, in essence, the last line of U.S. defense
 against an attack by an intercontinental-range missile.
The Pentagon has other elements of missile defense that 
have shown to be more reliable, although they are designed
 to work against medium-range or shorter-range ballistic
 missiles. These include the Patriot missile, which numerous
 countries have purchased from the U.S., and the Terminal
 High-Altitude Area Defense, or THAAD, which the U.S.
 deployed this year to South Korea to defend against 
medium-range missiles from North Korea.