Cook also noted that the Defense Department did not believe North Korea was able yet to sufficiently miniaturize any nuclear device to use it as a warhead on any missile.
"The United States has not seen North Korea demonstrate an ability to miniaturize a warhead. Nothing has changed… Our assessment has not changed," Cook stated.
Since the beginning of 2016, North Korea has conducted a nuclear weapon test, its fourth to date, and launched a long-range rocket, defying a UN Security Council resolution that bans Pyongyang from launching rockets that could be used as ballistic missiles carrying nuclear warheads.
On March 4, South Korea and the United States started discussing the US-proposed deployment of the Terminal High Altitude Air Defense (THAAD) system in talks in Seoul, according to the South Korean defense ministry.