Japan should show leadership by setting mid-term targets for cutting greenhouse gas emissions, the European Commission's top climate change negotiator said.
Artur Runge-Metzger was speaking in Tokyo after attending an informal dialogue among senior climate change negotiators.
"It is now up to the political leadership in Japan to take the next step and come up with an ambitious target and put it on the table," he told reporters.
"That certainly will create a lot of confidence around the table, among developed countries, but also on the side of developing countries so that we can move the negotiations forward," he added.
Prime Minister Taro Aso has said Japan will unveil its mid-term targets, which indicate goals for 2020, by June.
The government has already pledged to cut greenhouse gas emissions 60-80 per cent from 2005 levels by 2050, but has come under increasing pressure to commit to a mid-term target as a sign of Japan's willingness to do its part to fight climate change.
Developing nations, which blame industrialised countries for emitting most of mankind's greenhouse gas pollution, also want pledges for a massive jump in funding to pay for adaptation to climate change impacts as well transferring clean-energy technology.
About 190 nations are meeting at the end of the year in the Danish capital Copenhagen to try to agree on a broader successor to the Kyoto Protocol, the UN's main weapon in the fight against climate change. Japan's top climate advisory panel is considering a wide range of options, the least ambitious of which would allow a six per cent rise in emissions compared with 1990 levels.
That compares with Japan's existing commitment under the Kyoto Protocol to cut emissions by six per cent from 1990 levels during the pact's 2008-2012 first commitment phase. Japan is already way above that target and is buying UN-backed carbon offsets to try to meet its obligations under Kyoto.
Among the toughest mid-term options would be to cut emissions by 25 per cent from 1990 levels by 2020, by introducing a range of policies including a massive increase in the use of solar power.
The UN's climate chief, Yvo de Boer, speaking after the closed-door meeting in Tokyo, said it was important not to be fixated by the size of the mid-term target.
"A small number does not necessarily mean a small effort and a big number does not necessarily mean a big effort. Japan already made huge efforts including under the Kyoto Protocol to reduce emissions," Mr de Boer told reporters.
Mr Runge-Metzger said he wanted to take a closer look at Japan's scenarios before commenting on them. But he emphasised there were many issues to be settled before the UN conference in Copenhagen in December.
"Time is short," he said. "There is an enormous amount of work to be done in the coming months."