Russian President Dmitry Medvedev has called for new international rules covering safety at nuclear plants.
Such rules would permit the "necessary" development of nuclear energy, he said.
Mr Medvedev was making an unprecedented visit to Chernobyl, alongside Ukrainian President Viktor Yanukovych, to mark the 25th anniversary of the world's worst nuclear accident.
The two men attended a commemoration ceremony at the plant where a reactor exploded in 1986.
The explosion sent a plume of radiation across Ukraine, Belarus, western Russia and other parts of Europe in 1986.
Thirty people were killed directly by the disaster and more than 100 received radiation injuries, while thousands of cases of child thyroid cancer in the region may be linked to Chernobyl.
The anniversary comes amid renewed global protest over nuclear power. Thousands of people took part in protests in France and Germany on Monday, with demonstrations of hundreds reported in Lithuania and Romania on Tuesday.
The debate has been reinvigorated by the threat of radiation from Japan's crippled Fukushima plant in the aftermath of a devastating earthquake and tsunami.
Pledge of honesty
"Today, I sent proposals to [world] leaders... aimed at guaranteeing the necessary development of nuclear energy in the world while preventing at the same time catastrophic global consequences [of accidents]," Mr Medvedev said in remarks as he stood in spring sunshine in front of the hulk of the disused plant, according to Reuters.
He did not specify what the proposals were.
Soviet officials held off reporting the accident for several days, and Mr Medvedev said the disaster had taught nations of the importance of telling the truth to their people.
"The duty of a state is to tell the truth to its people. It must be acknowledged that the [Soviet] state did not always behave correctly," he said.
"In order for such tragedies never to be repeated we must all be honest, we must provide absolutely exact information about what is going on."
The call for honesty was welcomed by some 3,000 Chernobyl victims who joined a memorial service at a monument in Kiev.
They complain that benefits packages for workers made ill by participating in the clean-up have been cut in recent years.
Mr Yanukovych also stressed the need for global co-operation in nuclear safety, saying: "Chernobyl was a challenge of planetary dimensions. The answer to this challenge can be provided only by the world community."
Soviet engineers encased the damaged reactor in a temporary concrete casing to limit the radiation but a new shield is now needed.
A donor conference in Kiev, Ukraine, last week raised 550m euros (£486m; $798m) of the 740m euros needed to build a new shelter and a storage facility for spent fuel.
The two men took part in a religious memorial service led by Orthodox Patriarch Kirill near the plant, placed the first stone of a monument to clean-up workers and laid flowers at another monument.
Belarusian President Alexander Lukashenko did not take part in events at Chernobyl.
His office was earlier reported as saying he would be taking part in services within Belarus but when questioned by reporters, he rounded on the Ukrainian president with fury, describing Ukraine's leadership as "rather lousy".
According to a UN report this year, considered the most comprehensive to date on the disaster's health impact, more than 6,000 cases of thyroid cancer were reported in children and adolescents who were exposed at the time of the accident in Ukraine and Russia.
Many of these cancers were "most likely caused by radiation exposures shortly after the accident".
"Apart from this increase, there is no evidence of a major public health impact attributable to radiation exposure two decades after the accident," the report adds.