President Barack Obama on Monday dismissed North Korea's release of two imprisoned Americans as "small gestures", saying nuclear-armed Pyongyang must change its attitude on atomic weapons if it wants improved ties.

"We have been consistent in saying that when and if North Korea becomes serious about denuclearisation on the peninsula and is prepared to have a conversation around that topic, then the United States is going to be very open to try to arrive at a solution," Obama told reporters in Beijing.

"Until that time, there is going to be a core problem between us," he added, speaking during a meeting with Australian Prime Minister Tony Abbott.

Both Obama and Abbott are in China to attend the Asia-Pacific Economic Cooperation forum annual summit.

Obama said that while he was happy for the families of the freed Americans, it was going to take more than "small gestures like the ones that we saw, the release of these individuals" for relations to improve.

"Until this point we have not seen serious engagement on the part of Pyongyang to deal with that problem," he said, referring to the nuclear issue.

US citizens Kenneth Bae and Matthew Miller were released after a secret mission to North Korea by US intelligence chief James Clapper.

Clapper spent less than a day in Pyongyang, conducting talks with senior offices but did not meet leader Kim Jong-Un, before flying out with the two freed men to an airbase in Washington state on Saturday, a US senior administration official said.

Obama elaborated on Clapper's mission, saying issues such as Pyongyang's atomic programme were not discussed.

"It did not touch on some of the broader issues that have been the source of primary concern when it comes to North Korea, in particular, its development of nuclear capacity," Obama said.

North Korea has expressed interest in reviving long-stalled six-party talks with the US and others on the nuclear issue, but Washington insists Pyongyang must first show a tangible commitment to denuclearisation.

No "high level policy discussions between Jim Clapper and the North Koreans" took place, Obama said.

Bae, a Korean-American, was detained for two years. The 46-year-old was arrested in November 2012 and later sentenced to 15 years' hard labour for "hostile acts" against North Korea, having been accused of trying to "topple" the hermit kingdom.

According to his family, Bae was held while leading a tour group as the owner of a travel agency he ran in neighbouring China.

Miller, 24, was sentenced to six years hard labour by the North Korean Supreme Court following his arrest in April after he allegedly ripped up his visa at immigration and demanded asylum.