RIO DE JANEIRO, June 18 (Xinhua) -- With only two days to go before the opening of a major UN conference on sustainable development, participating countries are still far from reaching a political consensus, a Chinese expert said here Sunday.

The United Nations Conference on Sustainable Development (Rio+20 Earth Summit) will take place here from June 20 to 22, aiming to define pathways to a safer, more equitable, cleaner, greener and more prosperous world for all.

"Negotiations are still relatively hard as different parties are more insistent on their positions," said Zhou Dadi, researcher of China's Energy Research Institute and a leading expert of the Chinese delegation.

Now the responsibility for the negotiations has been handed over to the host country, Brazil, Zhou said, adding that a draft document of the final political outcome is prepared but may take longer time than expected.

Although all sides support sustainable development and green economy, difficulties in the negotiation process lie in how to apply the principle of "common but differentiated responsibilities" and whether developed countries could assume due responsibilities rather than transfer the burden of green economy to developing countries, he said.

In addition, disagreements on a number of issues such as green economy and sustainable development objectives also reflected conflicts of interests among participating countries, which are difficult to bridge, he said.

"Green economy can not become trade and technical barriers, which will cause even greater disadvantage to developing countries," Zhou said, citing the European Union's aviation carbon tax as an example.

Therefore, every country should be allowed to choose the most suitable development path of green economy according to different development stages and national conditions, he said.

The Rio+20 summit, the largest UN conference ever to be held here, will bring together leaders from more than 190 nations, who are seeking to realize a series of political agreements to improve people's living standard and protect the environment.

Twenty years after the 1992 Earth Summit in Rio, the United Nations is again bringing together governments, international institutions and major groups to discuss a range of measures to reduce poverty, promote decent jobs, clean energy and a more sustainable and fairer use of resources.