The head of the UN atomic agency called Monday on countries with nuclear power not to lose their "sense of urgency" in improving safety after last year's Fukushima disaster in Japan.

Opening a conference in Vienna analysing the aftermath of the world's worst nuclear accident in 25 years, International Atomic Energy Agency head Yukiya Amano said "significant progress" had been made since.

This included implementing measures outlined in the IAEA's Action Plan drawn up after Fukushima such as assessing possible risks, improving readiness for emergencies, reviewing safety standards and convening expert meetings.

"The accident may have faded from the international headlines, but it is essential that all of us ... maintain our sense of urgency and our commitment to implementing the Action Plan in full," Amano said.

"Much work remains to be done and we must not relax our guard."

Attended by around 700 delegates, the five-day conference at IAEA headquarters brings together the 75 states that are party to the 1994 Convention on Nuclear Safety (CNS).

A massive earthquake off Japan in March 2011 created a tsunami that swamped the Fukushima Daiichi plant, sending three reactors into meltdown, spewing radiation over a large area and forcing the evacuation of thousands of people.