SENDAI, Japan — U.S. naval barges loaded with fresh water 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 Fukushima Dai-ichi plant have been using seawater in a frantic bid to stabilize overheating reactors since a tsunami knocked out the complex’s cooling system March 11, but fears are mounting about the corrosive nature of the salt in the water.

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

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.

The switch to freshwater was the latest tactic in efforts to gain control of the nuclear plant 140 miles 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.

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.