By Fredrik Dahl and Sylvia Westall
VIENNA (Reuters) - Possible damage to the core of a disaster-stricken reactor and other worrying developments make it difficult to predict how Japan's nuclear emergency will develop, the U.N. atomic watchdog chief said on Tuesday.
Yukiya Amano, director general of the International Atomic Energy Agency, said he wanted more timely and detailed information from Japan -- his first hint at frustration with the pace of updates from authorities in his home country.
"The problem is very complicated, we do not have all the details of the information so what we can do is limited," Amano told a news conference. "I am trying to further improve the communication."
Japanese media have criticized the government's handling of the disaster and nuclear plant operator Tokyo Electric Power Co (TEPCO) for its failure to provide enough information on the incident.
Amano said the Vienna-based U.N. agency planned to send a small team of experts to Japan, possibly to help with environmental monitoring.
He spoke as Japan's crisis appeared to escalate when the operators of the Fukushima plant said one of two blasts had blown a hole in the building housing its No.4 reactor, which meant spent nuclear fuel was exposed to the atmosphere.
Amano said a storage pond with almost 800 spent fuel assemblies -- which are highly radioactive -- caught fire for about two hours before the blaze was extinguished.
There was also possible core damage in the No. 2 reactor, estimated at less than 5 percent of the fuel, and there might also be damage to the unit's primary containment structure.
"Is it a crack? Is it a hole? Is it nothing? That we don't know yet," Amano said.
But he said the pressure in the containment vessel had not fallen. "If there is a huge damage the pressure should go down."
The nuclear accident in which cooling systems broke down after the quake and tsunami, was the worst since the Chernobyl disaster in Ukraine in 1986, provoking fears of a fuel meltdown and a major radiation leak.
In Chernobyl, the explosion took place because the nuclear reaction was not halted, unlike in Fukushima, where the reactor was automatically shut down when the earthquake occurred.
Amano said he still believed the situation was different from that of the Chernobyl disaster, even though recent events "are worrying" and around 200,000 people had been evacuated from around the affected areas.
Japanese authorities have spent days trying to prevent the water which is designed to cool the radioactive cores of the reactors from running dry, overheating and emitting dangerous radioactive materials.
"It is difficult to foresee whether the future developments will be a worsening or improving of the situation," Amano said. "We do not know. There are mixed indications."