* Nuclear watchdog reports "good progress" in clean-up
* Still, Fukushima plant recently leaked radioactive water (Adds Amano quotes from report)
VIENNA, May 14 (Reuters) - The U.N. nuclear watchdog said the management of radioactive waste and contaminated water at Japan's tsunami-crippled Fukushima nuclear power plant could be improved despite good progress in cleaning up the site.
The operator of the plant said in February it had found a pool of highly contaminated water on the roof of a plant building and that it had probably leaked into the sea through a gutter when it rained.
A massive earthquake and tsunami four years ago caused meltdowns at Tokyo Electric Power Co Inc's (TEPCO) Fukushima reactors.
Some of the leaks have been dealt with to avoid a repetition of such incidents, the International Atomic Energy Agency (IAEA) said. Measures have included filling and covering of gaps, recovery of contaminated soil and treating surfaces to prevent rainwater leakages.
"While acknowledging these efforts, the IAEA experts encourage TEPCO to continue to focus on finding any other sources contaminating the channels," the agency said in a statement on Thursday.
The creation in 2014 of a new branch of TEPCO, called Fukushima Daiichi Decontamination and Decommissioning Engineering Company (FDEC), was a good step to define clear responsibility for the clear-up, the IAEA said.
Still, there is room for improvement in how the body handles radioactive waste, for example by more complete waste characterisation and packaging, the U.N. watchdog added.
"FDEC could better employ long-term radioactive waste management principles (beyond the segregation, relocation and dose reduction/shielding currently performed)," the agency said.
"While recognising the usefulness of the large number of water treatment systems deployed by TEPCO for decontaminating and thereby ensuring highly radioactive water ... is not inappropriately released..., the IAEA team also notes that currently not all of these systems are operating to their full design capacity and performance."
The IAEA will send a team to Japan this month to collect water samples from the sea near the Fukushima plant to help Japanese authorities with radioactive data collection and analysis.
In a 240-page report released to the agency's member states on Thursday, IAEA chief Yukiya Amano said Japan had not been sufficiently prepared for the 2011 accident, which triggered action to improve safety at nuclear plants across the globe.
"Responsibilities were divided among a number of bodies and it was not always clear where authority lay," Amano said of Japan. "There were also certain weaknesses in plant design, in emergency preparedness and response arrangements and in planning for the management of a severe accident." (Reporting by Shadia Nasralla; Editing by Mark Heinrich)