Protesters condemn the US withdrawal from Kyoto
The agreement, which commits industrialised countries to reduce greenhouse gas emissions, was thrown into disarray when it was rejected by US President George W Bush in March.
Dutch Environment Minister Jan Pronk, who opened the talks in Bonn, said: "Do not start all over again. Do not waste our past efforts.
"(Kyoto) is the only game in town. It is the best we have."
Gloomy predictions
About 2,000 police will be protecting the summit with thousands of demonstrators expected, some on their way to the G8 summit in Genoa.
Despite gloomy predictions, there were some signs on the eve of the four-day summit that it may be possible to agree a way forward.
"My hopes are growing day by day," Mr Pronk said after three days of closed-door bargaining by officials.
He stressed that the talks were a resumption of those which had broken down in The Hague last November - not a new set of negotiations.
But according to BBC News Online's environment correspondent Alex Kirby, who is attending the talks, Mr Pronk's comments contrast with those of many delegates and observers.
A mood of resigned realism now seems to be stalking the corridors.
Deal hopes
Phil Clapp, of the National Environmental Trust, a US campaign group, told BBC News Online: "Two things have made people think: the collapse of the last round of talks in The Hague last November, and President Bush's repudiation of the protocol.
"What they're saying now is: 'It won't matter too much if we have to relax the targets, settle perhaps for cuts a few per cent smaller than we wanted. The important thing is to get a deal.'"
Some delegates are now speaking of settling for what they can get in the hope of building on any agreement in the future - thus offending as few people as possible, and pleasing nobody very much, says our correspondent.
Japan is unwilling to ratify Kyoto without the support of its close ally the US and is trying to extract far-reaching concessions as the price for its adherence.
It is pressing to be allowed to make more use than other countries of what are called "carbon sinks" - forests, grassland and other vegetation which absorbs carbon dioxide, and can be counted against a country's emissions reduction target.
It is also resisting all the proposals currently on the table about securing compliance with the protocol.
And unless Japan agrees to ratify Kyoto, there will be too few of the main polluting countries supporting it to make it work.
Three other countries, Russia, Canada and Australia, also want to reopen the argument about the use of nuclear power in combating global warming, an issue on which The Hague meeting reached broad agreement.