In an interview with BBC, Fallon said North Korea's nuclear program must be “halted” before it develops a missile capable of reaching London.
Asked if Pyongyang has the capability to hit the UK capital, the defense secretary said, “Not yet, but they are clearly accelerating the missile program, the range is getting longer and longer.”
“We have to get this program halted, because the dangers now of miscalculation, or some accident triggering a response, are extremely great.”
A week ago, North Korea carried out its sixth and largest nuclear test that further escalated tensions on the Korean Peninsula. The country said its hydrogen bomb could be placed on an intercontinental ballistic missile.
US President Donald Trump warned that the US was ready to use the “full range” of capabilities, including nuclear arsenal, at its disposal in dealing with North Korea.
Defense Secretary James Mattis also warned of “a massive military response” if there was any threat to the United States, its territories or its allies.
Asked whether the UK and other NATO members would help the United States if North Korea carried out an attack against Guam in the Pacific, Fallon said, “Guam is part of the United States.”
“It's United States sovereign territory and the United States, of course, under the United Nations, has the right to ask other members of the United Nations to join in its self-defense.”
The comments came a few weeks after the British government said it would not help the US target North Korea’s nuclear weapon sites.
President Trump has ordered the US military to shoot down any missile launched from North Korea and heading toward Guam or the US mainland.
Pyongyang says it will not give up on its nuclear deterrence unless Washington ends its hostile policy toward the country and dissolves the US-led UN command in South Korea. Thousands of US soldiers are stationed in South Korea and Japan.