The United States is not fully capable of protecting the homeland from ballistic missile attacks from Iran and North Korea despite spending billions of taxpayers' dollars on building missile defence system, a new congressional watchdog report suggests.

The study released last week by the General Accountability Office (GAO) indicates that the U.S. Missile Defence Agency (MDA) has not proven that it can defend the homeland against current missile defence threats and has yet to prove the efficiency of the system called Ground-Based Midcourse Defence.''

"The system has only shown a partial capability [to defend] against small numbers of simple ballistic missiles... There currently is no proof that the system can intercept a target representative of an intercontinental ballistic missile,'' the GAO report says, according to the Washington Free Beacon.

"It also remains to be seen if the defensive system is capable of performing a salvo intercept where two or more interceptors are utilised against a single threat,'' it adds.

The government report also says that flight testing, to date, was insufficient to demonstrate that an operationally useful defence capability exists.

"The agency has not demonstrated several key homeland missile defence capabilities and is relying on high-risk acquisition practices to achieve its goal of fielding 44 interceptors by the end of 2017,'' it says.

The MDA also failed to prove that the system can carry out its most critical responsibilities when it comes to intercepting ballistic missile strikes, the report points out.

The GAO report states that while the defence department has complied with a mandate to regularly provide updates about its progress on the missile system, it has "failed to explain how the system will improved in the coming years."

The Pentagon has likewise failed to submit the needed requirements for evaluating options for improving homeland missile defence, it says.

The MDA is "relying on a highly optimistic, aggressive schedule that overlaps development and testing with production activities, compromises reliability, extends risk to the warfighter, and risks the efficacy of flight testing," says the report.

The U.S. Defense Department is claiming that the current system is expected to defend against "small numbers of simple ballistic missile threats launched from North Korea and Iran.'' But it also admits that flaws exist and that tens of billions of dollars more are needed to redesign the system to improve function.

Officials tasked with evaluating the system said their assessment of the system's capabilities "remained unchanged,'' said Newsmax.