North Korea has successfully produced a miniaturized nuclear warhead that can fit inside its missiles, crossing a key threshold on the path to becoming a full-fledged nuclear power, U.S. intelligence officials have concluded in a confidential assessment.

The analysis, completed last month by the Defense Intelligence Agency, comes on the heels of another intelligence assessment that sharply raises the official estimate for the total number of bombs in the communist country’s atomic arsenal. The United States calculated last month that up to 60 nuclear weapons are now controlled by North Korean leader Kim Jong Un. Some independent experts think the number is much smaller.

The findings are likely to deepen concerns about an evolving North Korean military threat that appears to be advancing far more rapidly than many experts had predicted. U.S. officials concluded last month that Pyongyang is also outpacing expectations in its effort to build an intercontinental ballistic missile capable of striking the American mainland.