U.S. naval barges loaded with freshwater sped toward Japan's overheated nuclear plant on Saturday to help workers struggling to stem a worrying rise in radioactivity and remove dangerously contaminated water from the facility.

Workers at the stricken Fukushima Dai-ichi plant have been using seawater in a frantic bid to stabilize reactors overheating since a tsunami knocked out the complex's crucial cooling system March 11, but fears are mounting about the corrosive nature of the salt in the water.

Plant operator Tokyo Electric Power Co. is now rushing to inject the reactors with freshwater instead to prevent pipes from clogging and to begin extracting the radioactive water, Hidehiko Nishiyama of Japan's Nuclear and Industrial Safety Agency said Saturday.

The situation at the stricken plant remains unpredictable, government spokesman Yukio Edano said Saturday, adding that it would be "a long time" until the crisis is over.

"We seem to be keeping the situation from turning worse," he said. "But we still cannot be optimistic."

The switch to freshwater was the latest tactic in efforts to gain control of the six-unit nuclear power plant located 140 miles (220 kilometers) northeast of Tokyo.

The switch was necessary because of concerns that salt and other contaminants in seawater were clogging pipes and coating the surface of reactor vessels and fuel rods, hampering the cooling process, NISA said.

Defense Minister Yoshimi Kitazawa said late Friday that the U.S. government had made "an extremely urgent" request to switch to freshwater. He said the U.S. military was sending water to nearby Onahama Bay and that water injections could begin early next week.

The U.S. 7th Fleet confirmed that barges loaded with 500,000 gallons of freshwater supplies were dispatched to the Fukushima plant.

Radiation has been seeping from the plant since a magnitude-9 earthquake and an ensuing tsunami on March 11, making its way into milk, seawater and 11 kinds of vegetables, including broccoli, cauliflower and turnips.