"North Korea's ballistic and nuclear tests are an affront to the United Nations Security Council," North Atlantic Treaty Organization Secretary General Jens Stoltenberg said in a joint announcement in Tokyo, where he met Japanese Prime Minister Shinzo Abe.
"They pose a global threat which requires a global response," Stoltenberg said.
Pyongyang has undertaken an unprecedented missile testing programme in recent months, as well as its biggest nuclear test yet in early September.
Stoltenberg's trip to Tokyo comes ahead of US President Donald Trump's first official visit to Asia, which starts in Japan on Sunday.
Talks about North Korea are expected to dominate a trip that will include a stop in Beijing, where President Trump is expected to pressure China's leadership to act more decisively against Pyongyang. China is North Korea's major ally and trading partner.
"Rather than dialogue it is necessary to cause North Korea to change policies by raising the pressure to the maximum," Abe said in a joint announcement with Stoltenberg.
President Trump has warned North Korea it would be totally destroyed if it threatened the United States with ballistic missiles and nuclear weapons. North Korea stoked tensions last week when it reiterated a threat to conduct an atmospheric nuclear test over the Pacific Ocean in a demonstration of its nuclear capability.