The two countries, Italy and Hungary, on Monday warned that the deal had to be followed by deeper cuts in greenhouse-gas pollution to avoid damaging the world's climate.
"We would have liked to announce and welcome here all parties for the first meeting of the Kyoto Protocol. Unfortunately, that is not possible," said Italian Environment Minister Altero Matteoli.
He was opening a conference in the Italian city of Milan that delegates from 180 countries were expected to attend.
"We must reinforce global strategies for the reduction of greenhouse gas emissions and for adapting to climate change for the most vulnerable regions of the planet," Matteoli said.
Hungarian Environment Minister Miklos Persanyi said the importance of the protocol had been made clear by the support it had received from around the world.
"The link between the higher concentration of these gases in the atmosphere and human activities causing these emissions is already unquestionable," said Persanyi, sworn in as annual president of the UN negotiations.
Kyoto requires industrialised signatories to cut emissions of six types of carbon gases, mainly emitted by fossil fuels, which trap the sun's heat, thus warming the atmosphere and nourishing a potentially catastrophic climate change.
The agreement was forced into intensive care after the United States, the world's biggest single polluter, withdrew from it in March 2001 in one of President George Bush's first acts in office.
"We can't afford to wait any longer"
Even though its complex rulebook has now been completed, Russia has yet to ratify the agreement and remains ambiguous over its intentions of doing so.
Moscow's ratification is needed to transform the protocol into an international treaty, giving a huge fillip to the campaign to combat climate change.
Even though Kyoto's political future is still unclear, man-made global warming is no longer in doubt among mainstream scientists, and more and more businesses are taking an interest in it, sensing a potential for profit.
"We can't afford to wait any longer," Steve Guilbeault of Greenpeace International said.
"Ministers at this meeting must speed up the process and make sure that practical measures to reduce greenhouse gas emissions are put in place," he said.
Although no declaration is due to be released when the meeting ends on 12 December, the forum should provide a pointer to Kyoto's chances of survival.