Weak public support for Japan's prime minister is making bold diplomacy tough, but Tokyo should still strive to resolve a decades-old territorial feud with Moscow to benefit both sides, a senior government adviser said.

Japanese politics has been all but paralysed by a deadlocked parliament, where opposition parties control the upper house and can delay bills.

Unpopular Prime Minister Taro Aso has been struggling to keep his job ahead of an election that must be held by October, although the opposition now has its own problems after a funding scandal erupted.

"Of course, if we have a very strong government, particularly with strong support from the people, it is easier for us to promote our diplomacy in a drastic way," Shotaro Yachi, a special government envoy, told Reuters in an interview on Wednesday.

"If it (the government) doesn't have a strong grounding ... we can't be so ambitious," he said.

Aso and Russian President Dmitry Medvedev made scant progress on resolving the dispute over the islands, known in Russia as the Southern Kuriles and in Japan as the Northern Territories, at a leaders' summit in Sakhalin last month at which they stressed growing economic ties between the two nations.

But Yachi said it was vital to keep trying to resolve the feud, which has prevented the two countries from signing a formal treaty ending World War Two.

"Necessary talks or dialogue must be maintained or even promoted, and if we accumulate dialogue with Russia, it might be helpful for the next administration or next, next administration," Yachi said.

SOVEREIGNTY HURDLE

Both sides claim sovereignty over the sparsely populated islands, the closest of which is just 15 km (nine miles) from Japan's northernmost island of Hokkaido and which lie amid rich fishing grounds close to Russian oil and gas production regions.

But experts say Russia needs international partners, such as Japan and China, to cement its expanding influence in Asia's hungry energy markets.

Yachi said a planned visit by Russian Prime Minister Vladimir Putin tentatively set for May would probably be too soon for a breakthrough in the dispute.

The best way forward, he said, was to put the islands dispute in the context of a broader, strategic relationship that would benefit both sides through cooperation in fields such as energy and the environment.

But he declined to outline any specific proposals.

"If you can draw a big win-win picture with the Northern Territory problem as one of the elements, a solution ... is possible," he said.

Aso, whom some critics say is trying to fill his diplomatic schedule to bolster his rock-bottom support rates, will probably go to China later this month to meet Chinese leaders, Yachi said.

Among the topics to be discussed are stalled talks on joint development of gas fields in disputed areas of the East China Sea, cooperation in the fields of energy and the environment and North Korea's nuclear programmes, as well as Tokyo's concerns about Beijing's bulging defence budget. The two neighbours also have a feud of their own over disputed islands.