Tokyo said on Tuesday it was trying to reach some agreement on disputed gas fields in the East China Sea to coincide with Chinese Premier Wen Jiabao's fence-mending visit to Japan this month, but large obstacles remained.

"We expect some announcement at the time of the prime minister's meeting in Japan but I am not so .. optimistic, it's very difficult," Japanese Foreign Ministry spokesman Mitsuo Sakaba said.

The announcement followed a meeting between Japanese Foreign Minister Taro Aso and his Chinese counterpart Li Zhaoxing on the sidelines of the 14th summit of the South Asian Association for Regional Cooperation, or SAARC.

Sakaba said both sides had agreed to accelerate talks over the energy issue.

Wen is due in Tokyo on April 11, the first such visit in seven years and the latest sign of a thaw in relations between the two Asian neighbours.

Some experts say Beijing is keen to see progress in the gas field feud ahead of Wen's meeting with Prime Minister Shinzo Abe.

Abe has tried to improve relations with China, which chilled under his predecessor, Junichiro Koizumi. He agreed at an October meeting with Chinese President Hu Jintao that the two sides would try to expedite a solution to the gas field spat.

The rights to the gas fields have long been a source of tension. The two sides have talked about joint development but differ on how that should be done.

China's CNOOC Ltd. had been due to start production in 2006 despite repeated Japanese calls not to do so.

The firm has declined to confirm whether production was under way, although a company official who asked not to be identified told Reuters in February work was going ""according to plan".

The Chinese drilling equipment is in undisputed waters, but Japan has expressed concern that it and other Chinese explorations could drain resources from Japan's exclusive economic zone.