Laura Grego, a senior scientist at the Union of Concerned Scientists, said before the announcement that a successful intercept did not mean the missile defence system is fully ready to defend the US in combat.
She added how the test was carried out under an unusually thick veil of secrecy.
"Success is better than failure, but because of the secrecy I have no idea how high the bar was set," she said.
"How realistic was the test? The Pentagon had a very long way to go to demonstrate the system works in a real-world situation."
The Pentagon is putting additional billions of dollars into expanding its arsenal of missile interceptors, which are based mainly at Fort Greely in Alaska.
In the 2020 defence budget request, the Pentagon asked for 9.4 billion dollars (£7.1 billion) for missile defence, including the system based in Alaska.