North Korea has yet to develop a nuclear warhead small enough to fit on a missile, a senior US official said on Wednesday, contradicting a recent US military intelligence report.

The North claimed its third atomic test staged in February involved a "miniaturised and lighter" warhead, prompting speculation that it had acquired the crucial technology to fit nuclear devices to a missile delivery system.

The latest test - the North's most powerful to date - came only two months after it successfully launched a long-range rocket in what was widely viewed as a ballistic missile test.

"I don't believe they have the capability to miniaturise the nuclear warhead, put it on top of the missile, work the launch and reentry problem, and target," said the senior US official who declined to be identified.

Meanwhile, Japan's Prime Minister Shinzo Abe said Wednesday he may meet North Korean leader Kim Jong-Un if it could help resolve the longstanding issue of Pyongyang's kidnapping of Japanese citizens.

"If a summit meeting is deemed as an important means in considering ways to resolve the abduction issue, we must take it into consideration as a matter of course in negotiating with them," Abe told a parliamentary committee.

The prime minister, responding to a question from an opposition lawmaker, was speaking the day after his senior aide arrived in Pyongyang on a surprise visit.

But Abe refused to comment on the purpose of Isao Iijima's trip to the reclusive state.

When then-Japanese prime minister Junichiro Koizumi visited Pyongyang in 2002, North Korea admitted its agents kidnapped Japanese nationals in the 1970s and 1980s to train spies in Japanese language and customs.

Some of those snatched were allowed to return to Japan along with children who were born in the North, but Pyongyang said the rest of them had died.

However, many in Japan believe the North is still holding some and Pyongyang's perceived refusal to come clean has derailed efforts to normalise ties.

Iijima was also a senior aide to Koizumi and is known to have played a role in organising his trips to Pyongyang in 2002 and 2004 for talks with then-North Korean leader Kim Jong-Il. Abe accompanied Koizumi on the 2002 visit.

According to Japanese media, Iijima, who is seen as having cultivated his own connections in North Korea, was greeted at the airport in Pyongyang by Kim Chol-Ho, vice director of the foreign ministry's Asian affairs department.

Iijama Wednesday held talks with Kim Yong-Il, secretary of the central committee of the ruling Workers' Party of Korea, the North's state news agency said without elaborating.

Reports Wednesday were rife with speculation that the North was trying to thaw icy relations with Japan at a time when ties with the United States and South Korea have gone into deep freeze after nuclear and missile tests.

The US, along with its two Asian allies, has increased pressure on Pyongyang to drop its nuclear ambitions and to join the international community.

Beijing has also taken a firmer line with its sometimes wayward ally, offering rare public rebukes that analysts said revealed frustration at Kim Jong-Un's administration.

Despite his hawkish tone, Abe has shown a pragmatic side in foreign relations, reaching out to South Korea and China during his first brief stint as prime minister.

Chief Cabinet Secretary Yoshihide Suga, Japan's top government spokesman, declined to discuss Iijima's visit.

He said Tokyo stood firm on its mission to resolve the kidnapping issue as well as North Korea's military threats.

"Japan's North Korean policies are clear. With dialogue and pressure, we are working toward a comprehensive resolution of various issues such as abduction, the nuclear issue and missiles," Suga told a regular press briefing.

"Particularly, abduction is the priority subject that we as the government must resolve," he said.