The deadly explosion at the Soviet-era plant "marked the conscience of humanity" and "remains a warning of the risks inherent in the use of increasingly powerful technologies," the US pontiff said at the end of a prayer service at the Vatican.

"I hope that at all levels of decision-making, discernment and responsibility will always prevail, so that every use of atomic energy may be at the service of life and peace," the 70-year-old said.

The 1986 explosion at Chernobyl, in current day Ukraine, was the worst civilian nuclear disaster in history.

A 2005 UN report put the number of confirmed and projected deaths from radiation exposure at 4,000 in the three worst-affected countries. Other NGOs and bodies have given higher tolls.

Some 600,000 people involved in the clean-up operation -- known as "liquidators" -- were exposed to high levels of radiation.