The tests were of a missile reportedly larger than an ICBM North Korea launched in 2017 that was assessed to be capable of reaching the United States.
American missile defence and reconnaissance forces in the Pacific have been placed in a state of "enhanced readiness" in preparation for a full-range test, a senior administration official said.
The official outlined the US intelligence assessment of the recent launches on the condition of anonymity.
"The purpose of these tests, which did not demonstrate ICBM range, was likely to evaluate this new system before conducting a test at full range in the future, potentially disguised as a space launch," Pentagon press secretary John Kirby said on Thursday.
North Korea has claimed the March 4 and February 26 launches were merely to test cameras to be installed on a future spy satellite.
Multiple UN Security Council resolutions prohibit North Korea from firing ICBMs, and the US will announce a new round of sanctions on Friday meant to make it more difficult for the country to access technology needed for its weapons programs, the official said.
The 2017 launch was part of a series of tests that prompted then-president Donald Trump to threaten North Korea's leaders with "fire and fury" and brought the two countries to the brink of more serious conflict.
The new missile was first revealed to the public in 2020 during celebrations marking the 75th birthday of North Korea's Communist Party in Pyongyang.
It appeared to be an intercontinental ballistic missile that is larger than any of the North's known ICBMs. The US official who commented did not estimate the maximum range.
Last week's launch was North Korea's ninth round of weapons tests already this year as it uses a break in diplomacy to expand its military capabilities while attempting to pressure the United States for concessions.
The Biden administration has tried to bring North Korea back to the negotiating table after three rounds of Trump meetings with leader Kim Jong-un aimed at denuclearising the Korean peninsula brought no appreciable change in the North Korean posture.
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