A top US general said Thursday it was unclear why an interceptor meant to knock out incoming ballistic missiles failed in a test for the second time in a row.

General James Cartwright, the military's second highest ranking officer, said he had no doubts that the country's missile defense system could fend off potential threats, despite Wednesday's failed test.

"I'm not the least bit concerned that we don't have a capability to defeat, should we need to, that rogue threat that the system's been designed against," Cartwright told reporters.

In the test on Kwajalein Atoll in the Marshall Islands, an interceptor rocket launched from a base in California failed to intercept its target, an intermediate-range ballistic missile, the Missile Defense Agency said.

Officials will carry out an "extensive investigation to determine the cause of the failure to intercept the target," the agency said in a statement.

A similar test carried on January 31 resulted in failure.

Cartwright said the latest test involved an "upgraded version" of an interceptor, but that older models currently deployed were reliable.

"We test in order to find out if something works," Cartwright said.

"The question now is, make sure we understand, was it two failures that were of the same ilk or was it two very different failures?" he said.

"We just don't know those answers yet."

Out of 15 tests of ground-based interceptors since 1999, seven have failed, according to the Missile Defense Agency.

"The next flight test will be determined after identification of the cause of the failure," the agency said.

Aerospace giant Boeing is the lead contractor for the missile defense program.

The ground-based defense system is meant to shield the United States from a potential ballistic missile attack, possibly from North Korea or Iran.