KIEV (AP) ― The world must prepare for more nuclear accidents on the scale of Chernobyl and Japan’s Fukushima Daiichi plant, the U.N. chief warned, saying that grim reality will demand sharp improvements in international cooperation.

U.N. Secretary-General Ban Ki-moon and others portrayed the growth of nuclear power plants as inevitable in an energy-hungry world as they spoke at a Kiev conference Wednesday commemorating the explosion of a reactor at Ukraine’s Chernobyl nuclear reactor 25 years ago.

“To many, nuclear energy looks to be a relatively clean and logical choice in an era of increasing resource scarcity. Yet the record requires us to ask painful questions: Have we correctly calculated its risks and costs? Are we doing all we can to keep the world’s people safe?” Ban said. “The unfortunate truth is that we are likely to see more such disasters.”

During a brief visit to the explosion site 100 kilometers north of the Ukrainian capital earlier in the day, Ban proposed a strategy for improving nuclear energy security worldwide, including strengthening the International Atomic Energy Agency and devoting more attention to “the new nexus between natural disasters and nuclear safety.”

The ongoing crisis at Japan’s Fukushima Daiichi nuclear power plant was triggered by last month’s huge earthquake and the ensuing tsunami that flooded the plant.

“Climate change means more incidents of freak weather,” Ban said in Kiev. “Our vulnerability will only grow.”

IAEA head Yukiya Amano, who accompanied Ban on the trip to Chernobyl, echoed those sentiments.

“Many countries will continue to find nuclear power an important option in the future, and that is why we have to do our utmost to ensure safety,” he said, speaking a few hundred meters from the exploded reactor, which is now covered by a hastily erected sarcophagus.

The sarcophagus has gone past its expected service life and work has begun to build an enormous shelter that will be rolled over the reactor building. The new shelter, designed to last 100 years, is expected to be in place by 2015, but a substantial amount of money for the project is still lacking.

An international donors conference Tuesday in Kiev sought to raise $1.1 billion for the shelter and a storage facility for the spent fuel at the plant’s other decommissioned reactors. But in the face of global economic problems, some countries held back from making funding promises and the pledges only came to $798 million.

The Chernobyl explosion on April 26, 1986, spewed a cloud of radioactive fallout over much of Europe and forced hundreds of thousands from their homes in the most heavily hit areas. A 30-kilometer area radiating from the plant remains uninhabited except for some plant workers who rotate in and several hundred local people who returned to their homes despite official warnings.

Zsuzsanna Jacab of the U.N.’s World Health Organization told the Kiev conference that some 6,000 cases of thyroid cancer had been diagnosed among people who were children and teens when exposed to the fallout. She said more cases are expected although “the magnitude is difficult to quantify.”

Among the 600,000 people most heavily exposed to radiation ― which apparently include the estimated 240,000 who worked on the first and most dangerous phase of the plant repair and clean-up ― Jacab expects 4,000 more cancer deaths than average to be eventually found.

The U.N. figures have been criticized by the environmental group Greenpeace and others as severely understating Chernobyl’s consequences. Even the lower figures represent “an unacceptable price paid by the affected communities,” Jacab said.

Ban and others said the Chernobyl and Japan accidents highlighted the need for improved communication between countries about their nuclear programs. And Thorbjorn Jagland, secretary-general of the Council of Europe, drew a political lesson from the crises.

“The more complex technologies become, the more complex societies become, the more important it is to involve civil societies, to have democratic institutions, a free press,” he said.

Soviet authorities kept the Chernobyl disaster unreported for several days, and Japanese authorities have been criticized for initially providing insufficient information.