The US defence missile system has put a strain on US-Russian relations
A former top US official has told a Congressional hearing in Washington billions of dollars have been wasted on developing a US missile defence system.
Philip Coyle, who used to be in charge of weapons testing at the Pentagon, said the threat used to justify the system was exaggerated.
Another witness said the missile system offered no prospect of defending the US from a real attack.
The US says the system will protect against missiles states like Iran.
However, plans to site the shield in central Europe have soured US-Russian ties. Moscow fears it could be used against Russia.
"Missile defence is the most difficult development the Pentagon has ever attempted," said Mr Coyle.
The threat being used to justify the large sums being spent has been exaggerated, he said, "and if it were real the proposed missile defence systems couldn't deal with it anyway".
Another witness, Lisbeth Gronlund, co-director of the Union of Concerned Scientists' Global Security Program, told the hearing the US was no closer to being able to "effectively defend against long-range ballistic missiles than it was 25 years ago".
But other witnesses at the hearing said the system's capability would improve with further research and development.
Jeff Kueter, president of the private George C Marshall Institute that focuses on how science is used in making public policy, said he detected "significant progress" in building the shield since 2002.
"Further improvement of the defence is essential," he said.
President George W Bush is seeking more than $13bn (£6.5bn) for all missile defence programmes for next year and a projected total of more than $60bn over the coming five years, reports say.