The US says N Korea recently tested parts of a new intercontinental ballistic missile (ICBM) system, in what it called a "serious escalation".
Pyongyang said the launches on 26 February and 4 March focused on developing a reconnaissance satellite.
But the Pentagon now says the tests were experimental launches, before a likely full-range ICBM launch.
With a minimum range of 5,500km (3,417 miles), ICBMs can reach the US. They are designed for nuclear arms delivery.
On Thursday, Pentagon spokesman John Kirby said North Korea's two tests "involved a new intercontinental ballistic missile system".
Neither launch displayed ICBM range or capability, but the tests were carried out "to evaluate this new system before conducting a test at full range in the future, potentially disguised as a space launch," Mr Kirby said.
"The United States strongly condemns these launches, which are a brazen violation of multiple United Nations Security Council resolutions, needlessly raise tensions and risk destabilising the security situation in the region."
Both South Korea and Japan have confirmed the US statement and condemned Pyongyang.
