The EU has offered Japan a package of incentives to persuade it to give up its bid to host the world's biggest nuclear fusion reactor.
ITER - NUCLEAR FUSION PROJECT
Project estimated to cost 10bn euros and will run for 35 years
It will produce the first sustained fusion reactions
Final stage before full prototype of commercial reactor is built
Europe wants to base the reactor at Cadarache in France, while Tokyo favours Rokkasho-Mura, in north Japan.
Japan would receive contributions to other fusion research initiatives and other benefits, EU officials said.
But Europe said it would press ahead with building the reactor itself if agreement was not reached soon.
"A central feature of a consensus would be a genuine partnership between the EU and Japan," the EU's executive commission said in a statement.
"Japan could receive favourable conditions to reflect its special contribution to the Iter project.
"Furthermore, the EU could contribute to other fusion research initiatives carried out in Japan to complement the Iter project as part of a broader approach to mastering fusion energy."
Breaking the deadlock
After the International Space Station, the multi-billion-dollar International Thermonuclear Experimental Reactor (Iter) would be the largest international research and development collaboration.
Unlike in fission reactions, in which atomic nuclei are split to release energy, fusion reactions release energy when nuclei are forced together.
The process is the same as the one that powers the Sun. Achieving stable and sustained reactions on Earth present an immense challenge, however.
Nonetheless, scientists believe they have learnt enough about the technical requirements over the past few decades to now push forward with a large-scale reactor.
If the technologies can be proven in Iter, the international community would then build a prototype commercial reactor, dubbed Demo.
European Commissioner for Research, Louis Michel, told a news conference he was optimistic that a deal among the six partners in the international project could be reached - but made clear that the EU was willing to proceed without those partners if necessary.
"I think there's a good chance to succeed with this project with six," he said. "But you never can be sure."
Talks last week between the EU, South Korea, Russia, the US, China and Japan on where to build the multi-billion-dollar Iter ended in deadlock.
EU sources say the European side is confident, largely because it is widely believed in Brussels that South Korea and the United States would be prepared to back Cadarache if Japan steps aside. China and Russia already favour the French site.
"If there is no agreement at six we are determined to do it with fewer," Mr Michel told reporters, adding that the EU would prefer to reach a deal with the current partners or, even better, more.
The EU wants an agreement on the project before the end of the year but has not set a deadline and would be willing to negotiate past December, Mr Michel added.
Japan has been irked by some of the comments in the media attributed to unnamed EU sources and believes talk of Brussels going it alone is unhelpful to the negotiations.
"(The EU's) negotiating stance is worrisome and regrettable," said Takahiro Hayashi, deputy director of Japan's Office of Fusion Energy. "I think it would be good to agree on a site quickly," he told the AFP news agency.