VIENNA - Japan's nuclear crisis has exposed huge weaknesses in how the world deals with such disasters, the UN nuclear chief said Monday, urging changes in emergency responses worldwide.

Yukiya Amano, head of the International Atomic Energy Agency, also told a 35-nation IAEA board meeting that -- while the situation at Japan's crippled Fukushima Dai-ichi nuclear site remains serious -- "we are starting to see some positive developments."

Pressed by reporters on whether his agency should be authorized by its 35-nation board to make IAEA safety standards mandatory instead of their present voluntary status, he said "there are some arguments" from board nations in favour -- but others were against.

"The vies are very different," he said indicating that any reforms will be slow in coming and less than optimally effective because of the need to achieve board consensus.

Japan's Fukushima Dai-ichi nuclear complex was crippled 10 days ago by a huge earthquake and massive tsunami, and Amano suggested that one area up for likely review is whether tsunami protection standards need to be strengthened.

But a comprehensive update of safety standards "needs more studies," he said -- again suggesting that any review will take time.

Inside the board meeting, he defended his agency's performance since the crisis broke, emphasizing that it is up to individual countries to focus on nuclear safety, with the IAEA only in an advisory role.

Glyn Davies, the chief U.S. delegate to the IAEA, however, suggested the agency needed to do more, in a joint U.S.-Canada statement that indicated agency board members will focus on more oversight of the organization's Japan performance.

He said the board will work with Amano "to ensure that this agency is bringing all of its resources to bear in addressing the current crisis."

Since a March 11 earthquake and tsunami knocked out the complex's power supplies, Fukushima's radioactive gas leaks have triggered the worst nuclear crisis since the 1986 Chernobyl disaster.

Japanese and IAEA officials have suggested the emergency might be slowly abating. But attention is now focusing on the plant's safety record, with Japan's nuclear safety agency criticizing the operator for repeatedly failing to inspect crucial equipment before the crisis broke.

Amano has repeatedly emphasized that the IAEA can only advise Japan and other member nations in such situations and has no mandate to enforce international or local regulations on nuclear safety.

"We are not a 'nuclear safety watchdog' and responsibility for nuclear safety lies with our member states," he said, in comments from the closed meeting provided to media. "In contrast to the agency's role in nuclear nonproliferation, nuclear safety measures are applied voluntarily by each individual country and our role is supportive."

His comments Monday contained no direct criticism of the way Tokyo has handled the emergency. Instead, Amano told the meeting: "I have confidence that the Japanese government will address public concerns properly."

Still, he touched on international allegations that Japan has been too slow in releasing information about conditions at the site and the dangers facing the Japanese public, as he called for revamping the way nations deal with future nuclear emergencies.

The present ways of responding to such disasters were based on lessons learned from Chernobyl and reflect "the realities of the 1980s, not of the 21st century," Amano said.

He acknowledged that his agency's "role in nuclear safety may need to be re-examined, along with the role of our safety standards," alluding to calls by member nations for a more muscular IAEA enforcement role.

"It is already clear that arrangements for putting international nuclear experts in touch with each other quickly during a crisis need to be improved," he added.

Davies, the chief U.S. delegate, also suggested a major nuclear review was needed, saying that the IAEA board planned to work with the agency and its member states to "act upon the lessons of this nuclear emergency."