The ambassador was speaking at the North Korean mission in Geneva on Friday, as South Korea and the U.S. agreed in Seoul to keep co-operating towards a peaceful end to the crisis.
North Korea has said it can not abandon its nuclear programme, which it says is necessary as a deterrent to a USA threat.
In recent months, it has fired two missiles over Japan and conducted its sixth and the largest nuclear test, stoking regional tension.
The commentary came out ahead of an imminent visit to Pyongyang later in the day by Chinese President Xi Jinping's special envoy Song Tao, the head of the worldwide department at the Central Committee of the Communist Party.
"China is sending an Envoy and Delegation to North Korea - A big move, we'll see what happens!" he tweeted. But he called on the North to stay that course "for a period of time", according to Yonhap. "Koreans-fire-at-soldier-trying-to-defect-to-South">North Korean soldier was shot six times by his former comrades as he attempted to make an escape through the Korean Demilitarized Zone and defect to the South.
While Washington has said that all options, including military, are on the table in dealing with North Korea, it has stressed its preference is for a diplomatic solution.More news: 1 militant killed, 3 arrested in ongoing Kund encounter
Mattis said the U.S. military was closely watching why there had been no missile launches recently, but declined to give potential reasons for it.
Mattis reportedly made the remarks to reporters as he was flying to Colorado to meet with officials of the North American Aerospace Defense Command and US Northern Command, on homeland and anti-missile defense. Beijing said Song will report the outcome of the party's latest congress to Pyongyang. "Developing friendly cooperation relations between the two countries serves the common interests of the two countries and is conducive to regional peace, stability and development", he added.
"As long as there is continuous hostile policy against my country by the United States and as long as there are continued war games at our doorstep, then there will not be negotiations", he said.
North Korea has been working to develop a nuclear-tipped missile capable of reaching the United States, sparking a major global crisis in which US President Donald Trump has said all options are under consideration, including military ones.
In the chairman's final statement on the recently concluded 31st Asean Summit in Manila, the regional bloc urged North Korea to "fully and immediately" follow the resolutions issued by the United Nations Security Council (UNCS), which demanded that Pyongyang cease its nuclear tests.