A system to treat contaminated water that is impeding repairs at Japan's damaged nuclear power plant is not performing as well as hoped but should be functioning fully next month, a government official said Thursday.

Since the March 11 tsunami, workers have cooled the reactors and spent fuel by pumping fresh water, which becomes contaminated with radiation. About 110,000 tons of tainted water have accumulated, threatening to leak into the sea and posing health risks and logistical hurdles to the workers struggling to make repairs at the plant.

"The contaminated water problem is the biggest barrier right now," said Goshi Hosono, director of the government's nuclear crisis task force. "We are anxious to stabilize the treatment system one way or the other."

Problems were expected along the way, he said, but added that he wanted to get the system fully operational by the end of June and running stably by mid-July.

"The water treatment system is a key step toward bringing the reactors to cold shutdowns," he said. The plant operator and the government's goal for cold shutdown is early January.

Workers at the Fukushima Dai-ichi plant have struggled to get the treatment system fully operational before the tainted water overflows. It's estimated that could happen in early July.

The contaminated water has hampered work to install a sustainable cooling system at the reactors. Unit 1 is close to that stage, but the other two reactors have fallen behind due to high radiation or large amounts of debris.

The water treatment system involves parts from around the world.

Toshiba Corp. and Hitachi have developed oil- and salt-removal components, but a trial run Wednesday of a component developed by Kurion Inc. of the United States only reduced the level of radioactive cesium to one-tenth of what had been expected, TEPCO spokesman Junichi Matsumoto said.

A component made by French nuclear giant Areva SA also underperformed, he said.

The treatment system was halted last week when a cartridge reached its limit of radioactive cesium after only five hours, not several weeks as expected. After cleaning and adjustments, the system is being tested and has intermittently processed nearly 2,000 tons of water, Matsumoto said.

Hosono said the system was assembled in a rush and problems were not surprising since the system has never been tested in an environment as severe as Fukushima Dai-ichi. He said he also has instructed TEPCO to secure extra tanks as emergency storage for the water to keep it from leaking into the ocean.