WASHINGTON - The US military will test-launch an intercontinental ballistic missile on Wednesday, the Pentagon announced, in the second such practice operation of the country's nuclear defence in less than a month.

"There will be an operational test launch of an Air Force Global Strike command unarmed Minuteman III intercontinental ballistic missile early tomorrow morning, Sept 7, from Vandenberg Space Force Base in California," Pentagon spokesman Brigadier-General Pat Ryder said on Tuesday.

The announcement ahead of the launch was unusual; the Pentagon has not confirmed recent tests until after they take place.

Ryder stressed the test would be "routine," adding that it had been long-scheduled and that the US had notified Russia and other countries of the plans.

The aim of the test "is to demonstrate the readiness of US nuclear forces and provide confidence in the security and effectiveness of the nation's nuclear deterrent," Ryder told reporters.

The US Air Force successfully launched a Minutemen III ICBM Aug 16, after having postponed the test twice to avoid stoking tensions over Ukraine and Taiwan.

The missile carried a test re-entry vehicle, which in a strategic conflict could be armed with a nuclear warhead.

The re-entry vehicle travelled about 4,200 miles (6,700km) to the Kwajalein Atoll in the Marshall Islands in the western Pacific.

Ryder said the two tests were scheduled long in advance and are occurring close together because of the first one's postponement. AFP