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His remarks during his trip to Asia which began on Tuesday come amid threats from Pyongyang that it is ready to conduct another nuclear test.
Intelligence reports cited by the media indicate that there has been increased activity in and around North Korea’s nuclear sites.
Obama warned that any such moves would meet with a firm response from the US, but he did not elaborate.
“North Korea has engaged in provocative actions for the last several decades,” he said during a joint news conference with his South Korean counterpart President Park Geun-hye.
“It’s been an irresponsible actor on the international stage for the last several decades. They are the most isolated country in the world. They are subject to more international sanctions and international condemnation than any country in the world,” he added.
While reaffirming that the US stands “shoulder to shoulder” with its South Korean ally, he earlier called on China to use its diplomatic channels to restrain North Korea.
US officials in previous years have said that China more than any other country has the greatest influence on North Korea.
For its part, North Korea has called Obama’s visit to Seoul “a provocation”. In recent weeks, it threatened to withdraw from talks designed to resume cross-border visits after Washington and Seoul went ahead with annually scheduled military exercises.
In previous announcements, North Korea has said ending the exercises was a precondition to reopening diplomatic channels to resume talks with the US and its southern neighbor.
North and South Korea are not technically “at peace” since no peace treaty was signed following the Korean War in 1953. The Demilitarized Zone between the countries is the most heavily armed border in the world.
In early September, the two Koreas reached an agreement to fully reopen the Kaesong industrial complex, which was shut down in April after tensions between Pyongyang, Seoul and Washington peaked over the former’s launching of upgraded missiles and the latter holding joint military exercises.
The Kaesong Industrial Complex was established in 2004 as an attempt to bring the two Koreas closer through cross-border cooperation and employing 53,000 North Korean workers.