"The situation in Fukushima remains very serious", Amano told a press briefing at UN headquarters here, where he is attending the Chief Executives Board meeting, which brings together heads of the world body's agencies.
"But Japan is not alone. The UN Secretary General promised me to give all the support necessary to Japan. The most important thing for now is to put an end to this crisis situation," he said.
Japan has struggled to contain its nuclear emergency since a 14-metre tsunami hit the Fukushima plant after a huge quake on March 11, with radioactive substances entering the air, sea and soil, plus foodstuffs from the region.
The disaster has left over 11,000 confirmed dead and more than 16,000 listed as missing.
Amano warned that ending the current crisis "will take some time" and that stabilising the stricken reactors "will take more time."
"And in order to say everything is normal, it will take further time .... I would say it would take more time than people think", he added.
Officials in Japan said Friday that the Fukushima Daiichi plant will use a huge steel floating structure to contain radioactive water it releases.
The pontoon-type structure, called a "Mega-Float," will be handed over to Tokyo Electric Power Co. (TEPCO) by its owner which has been using it as a floating park for anglers.
High levels of radioactive iodine-131 that are 10,000 times the government safety standard have been found in groundwater below the plant, located on the Pacific coast some 250 kilometres northeast of Tokyo.
In a stop-gap measure to contain the crisis at the plant, crews have poured thousands of tonnes of water onto reactors where fuel rods are thought to have partially melted, and topped up pools for spent fuel rods.
UN Secretary General Ban Ki-moon is also in Nairobi attending the board meeting.