VIENNA (AFP) - The United Nations atomic watchdog used the 30th anniversary on Tuesday (April 26) of the Chernobyl disaster, the world's worst nuclear accident, to warn against "complacency" in atomic safety.

"Chernobyl led to a leap forward in global cooperation on nuclear safety," International Atomic Energy Agency (IAEA) chief Yukiya Amano said in a statement.

Further improvements were made after the Fukushima disaster in Japan in 2011, but the "key lesson" from both events "is that safety can never be taken for granted".

"Complacency must be avoided at all costs," he said.

He said nuclear safety is primarily the responsibility of individual states, but that since accidents can transcend borders, effective international cooperation is "vital".

After Chernobyl, Mr Amano said, countries with nuclear power began sharing information and experience in a way they never had before. The IAEA's mandate on nuclear safety was enhanced and safety standards were expanded.

Important international legal instruments were adopted, including the Convention on Nuclear Safety (CNS), while an international coordinated response system is now in place, he said.

An IAEA "peer review" system has also been established, deploying international teams of experts to advise countries on the operational safety of their nuclear reactors or the effectiveness of their regulatory system.

Critics say however that no matter how many technical and regulatory improvements are made, nuclear power remains inherently dangerous, particularly with many reactors decades old, their designs dating back to the 1960s and 1970s.

"We are now in the wear-out stage for the majority of the reactors in the world," Mr Shawn-Patrick Stensil, a nuclear expert at Greenpeace, told AFP last week.

In addition, they were built before another risk that has reared its head in recent years - nuclear terrorism - "was even thought about", Mr Stensil said.