A senior U.S. Defense Department official says North Korea has significantly improved the capability of its offensive ballistic missiles.

The Director of the Defense Department's Missile Defense Agency, Ronald Kadish, said that although North Korea has adhered to a 1999 moratorium on flight testing its long-range missiles, they haven't stopped development.

Kadish said North Korea is working toward having a ballistic missile capability of all ranges across a broad front, but he didn't elaborate, saying details are classified.

But Joseph Cirincione, director for non-proliferation at the Carnegie Endowment for International Peace in Washington, said it's impossible to verify the accuracy of Kadish's assertion without access to intelligence.