The U.S. Navy is keeping all but critical missions from flying through a wide stretch of airspace above Japan's overheating nuclear plant — an even larger area than the U.S. government has recommended that Americans avoid on the ground.

The air cordon is intended to ensure safety and keep routine flights from passing through potentially hazardous airspace, Cmdr. Jeff Davis, a spokesman for the U.S. 7th Fleet, said Friday.

It's not a complete ban, with specific missions still being carried out inside a "heightened awareness zone," Davis said. U.S. aircraft are conducting flights to measure radiation coming from the Fukushima Dai-ichi nuclear plant, where reactors and fuel storage pools began overheating after the March 11 earthquake and tsunami knocked out cooling systems.

The restricted zone covers a radius of 100 nautical miles — or about 115 miles or 185 kilometers — above the Fukushima plant. The area is more than twice the 50-mile (80-kilometer) radius the U.S. government has urged Americans to avoid on the ground, which in turn is larger than the area where the Japanese government says people should either leave or stay indoors to prevent radiation exposure.

Setting a wider safety zone for Americans was seen as another rebuke of Japan's handling of the Fukushima crisis, which U.S. officials said was bleaker than the Japanese government has stated. The Navy this week widened its restricted flight zone first to 25 nautical miles (30 miles or 45 kilometers), then to 50 nautical miles (60 miles or 90 kilometers) and finally by Friday to 100 nautical miles.

Davis said missions inside the zone take place only after evaluating "current atmospheric conditions to include wind direction, and ensure that extra precautionary measures are being taken." He said crews are given potassium iodide, which retards absorption of radiation and reduces the risks of thyroid cancer, and are decontaminated if post-mission monitoring detects any contamination.