Labour and National governments have unsuccessfully tried to get Japan to join in the New Zealand-led provincial reconstruction team in Afghanistan's Bamyan province, confidential cables from the United States Embassy in Wellington show.

They suggest New Zealand thought Japan was afraid its troops would not perform well enough and was hiding behind its constitutional limitations - which prohibit any act of war. They also mention attempts to get Singapore and Malaysia involved.

New Zealand has a historic defence relationship with the latter two through the Five Power Defence Arrangement, which has been going for about 40 years.

But the cables, released by WikiLeaks, suggest New Zealand was trying to deepen defence ties with Japan, albeit unsuccessfully.

It is not clear whether the moves to lobby Japan were at the request of the US or New Zealand-initiated.

Defence Minister Wayne Mapp last night refused to discuss the matter, saying he did not conduct diplomacy through the media.

But a visiting high-ranking US official in April 2008, deputy assistant secretary Thomas Christensen, suggested that "any perceived enhancement of government of New Zealand-government of Japan relations will likely spur the Chinese to respond in kind to the Japanese ... so New Zealand can play a positive role in encouraging better Sino-Japanese relations by improving its own relationship with Japan".

The first reference in the cables to New Zealand's approach to Japan was dated August 2008. It concerned the visit of Major General Thomas Conant and Deputy Assistant Secretary of Defence James Clad.

The cable says that New Zealand defence official Paul Sinclair said that his government "has tried over the past nine months to engage the Japanese on participation in the New Zealand PRT in Bamyan".

"Sinclair observed that the government of Japan is sending some planes to participate in Operation Maru [an exercise in the Proliferation Security Initiative] but government of New Zealand efforts to build a defence relationship with Japan have been less than successful; NZ feels that Japanese forces lack self-confidence in overseas deployments. DASD Clad urged [New Zealand] to continue to engage with the Japanese."

The next reference was made in a cable in April last year when the Cabinet considered a call for a greater contribution. The cable is based on a briefing of embassy staff by Ministry of Defence and Ministry of Foreign Affairs and Trade officials.

"On encouraging the Japanese to provide more support in Afghanistan, MOD officials noted that the government of New Zealand has continued to raise Afghanistan in Japan-NZ bilateral discussions but to little avail.

"[NZ] has concluded that Japan's defence forces hide behind government of Japan constitutional limitations because - in NZ opinion - Japan's security forces lack self-confidence and worry that they may not be able to perform adequately."