VIENNA (Reuters) - The UN nuclear watchdog's chief on Monday called for a summit to strengthen nuclear safety and improve disaster management following Japan's crisis. Japan is struggling to avert a severe meltdown at its Fukushima Daiichi nuclear power plant and officials said on Monday highly radioactive water had been leaking from the site, hit by an earthquake and tsunami two weeks ago. IAEA chief Yukiya Amano said he wanted ministers from the International Atomic Energy Agency's 151 member states to attend the summit in Vienna, possibly to be held in June. "(The) political level is needed, this is a very important issue, this is not only for experts or technical people," he told a news conference. He described the situation at the site as "very serious.""The difficult situation has not been overcome and it will take time to stabilize the reactors. Radioactivity in the environment, foodstuffs and water is a matter of concern in the vicinity of the Fukushima plant and beyond," he said. The IAEA, tasked with promoting peaceful uses of atom energy, has been criticized in the media and privately by diplomats for being too slow to react to the crisis and communicate information. The agency has said it can only publish data given to it by Japan and says it lacks the power to enforce nuclear safety standards.