UN Secretary General Ban Ki-moon on Tuesday described North Korea's boast that it has missiles capable of reaching the US mainland as "alarming" and warned it would increase tension in the region.

Ban, a former foreign minister of South Korea, admitted he had no way of knowing if the claim issued by North Korea earlier on Tuesday was credible.

But in an interview with AFP, he said: "I read the report and it is quite an alarming statement by the DPRK (North Korea)."

"They should contribute to the reduction of tensions and this will only heighten tensions and create further distrust between South and North Korea," he added.

"Also, it is not the way to gain the trust of the international community. I sincerely hope that the DPRK authorities will sincerely engage in dialogue between the South and North as well as with neighbouring countries."

Ban said he concurred with the view of most analysts that the North Koreans were reacting to South Korea's announcement on Sunday of a new accord with the United States to almost triple the range of its missiles to 800 kilometres (500 miles).

The deal will give the South the capacity to hit targets across the North, as well as bringing parts of Japan and China into reach.

"Their reaction seems to come from the recent agreement between the Republic of Korea (South Korea) and the United States," Ban acknowledged. "However that is a totally separate issue."

Asked if he thought the military-dominated Communist regime in North Korea was blustering, Ban recalled the failure in April of a rocket launch that Pyongyang said was aimed at placing a satellite in orbit but the US and UN saw as an test for a potential intercontinental missile.

"Since the late 1990s they have been developing nuclear technology -- that is known fact," Ban added.

"They have failed in their test but I don't have any credible information about the level of their technology at this time."