North Korea has made a “miniaturized nuclear warhead that can fit inside its missiles,” The Washington Post reported Tuesday, citing a confidential intelligence assessment.

The development would mark a major step in the isolated nation’s push to become a nuclear power.

It comes after the Japanese Defense Ministry said it was possible that North Korea successfully miniaturized nuclear weapons, according to NBC News.

The U.N. Security Council on Saturday unanimously put new sanctions on North Korea over its continued missile tests.

Neither the Defense Intelligence Agency, which put together the analysis, nor the White House immediately responded to CNBC’s requests for comment.

Read the full Post report here.