VIENNA (Reuters) - Efforts to enhance global nuclear safety have advanced since Japan's Fukushima disaster struck exactly two years ago on Monday, the U.N. atomic energy chief said, vowing to help make nuclear power as "safe as humanly possible".

Yukiya Amano, a veteran Japanese diplomat, said virtually all member states of the United Nations' International Atomic Energy Agency (IAEA) with nuclear plants had completed so-called stress tests and had expanded safety measures.

"We must maintain the momentum of constant improvement", he said in a weekend statement to mark the anniversary of the world's worst nuclear accident in a quarter of a century.

Meltdowns at the Fukushima nuclear plant after the March 11, 2011, earthquake and tsunami sent radiation spewing over large areas, forcing more than 160,000 people to flee.

The worst such accident since Chernobyl also put a question mark over the future of nuclear energy elsewhere in the world. In Europe, Germany, Switzerland and Belgium decided to move away from nuclear to increase their reliance on renewable energy.

The IAEA has said it believes, however, that global use of nuclear energy could increase by as much as 100 percent by 2030 thanks to growth in Asia, including in China and India.

The Vienna-based agency was accused in 2011 of a slow initial response to the Fukushima disaster, but later led international efforts to agree an action plan to improve global reactor safety.

"The worst elements of the accident are behind us and we are now in the post-accident phase," Amano said. "The IAEA's 159 member states have already made significant progress in upgrading nuclear safety."

(Reporting by Fredrik Dahl; Editing by Alistair Lyon)