WASHINGTON (BLOOMBERG) - The Pentagon is about to attempt what's likely to be the most challenging test yet of the US military's ability to shoot down an incoming missile from an adversary such as North Korea or Iran, according to three people familiar with the plans.

The Missile Defence Agency intends on Monday (March 25) to fire two interceptors tipped with the latest Raytheon Co warheads within seconds of each other in a test that hasn't been publicly announced.

The first interceptor would attempt to crash into a dummy target representing an incoming intercontinental ballistic missile (ICBM). The second would use its sensors to detect another ICBM or other countermeasures.

Monday's action would be first missile defence test since a successful one in May 2017. The system also scored an interception in June 2014 after two that failed in 2010.

Mr Mark Wright, a spokesman for the Missile Defence Agency, said in an email said he had "nothing to announce at this time."

The US$36 billion (S$48.69 billion) system of Boeing Co-managed radar, command links and 44 ground-based interceptors in California and Alaska is designed to defeat a missile attack.

The Pentagon is requesting US$9.4 billion in fiscal 2020 for the agency's programs, including US$1.4 billion for the ground-based segment - an increase from the US$9.36 billion previously planned.

The interception attempt is the programme's first operational flight test and "if successful, will have demonstrated a fundamental, yet crucially important, aspect of how the war-fighter will operate" the system in a missile attack on the US homeland, said Ms Cristina Chaplain, space and missile defence systems director for the Government Accountability Office, in an e-mail.

"The test has been delayed by over a decade, as it may be the most challenging test in the program's near-30-year history," she said.