WASHINGTON — U.S. military relief crews are banned from going within 50 miles of Japan’s stricken nuclear power plant and are receiving anti-radiation pills before missions to areas of possible radiation exposure, the Pentagon said today.

With the arrival of three more ships to the massive humanitarian mission, there were 17,000 sailors and Marines afloat on 14 vessels in waters off Japan, Defense Department spokesman Col. Dave Lapan said. He didn’t have the number of U.S. forces participating from other service branches.

The operation is fraught with challenges, mainly how to continue to provide help amid some low-level releases of radiation from the troubled Fukushima Dai-ichi facility, which officials fear could be facing a meltdown.

Lapan said U.S. forces can’t go within 50 miles of the troubled plant without approval and that no one has yet been given approval. And the Navy has begun giving potassium iodide pills to helicopter crews that might be flying relief supplies or other missions into other areas where exposure is a risk.

Previously, only two people who had been exposed were given the pills, which can prevent the radioactive iodine type of radiation from being taken up by the thyroid gland.

Weather also temporarily hampered some relief plans on today. Pilots couldn’t fly helicopters off the deck of aircraft carrier USS Ronald Reagan until late afternoon because of poor visibility. The 7th Fleet said 15 flights with relief supplies were launched from the eight-ship carrier group, about half as many as the 29 flights reported the previous day to deliver food, water, blankets and other supplies.

Several water pumps and hoses were being sent from U.S. bases around Japan to help at Fukushima, where technicians were dousing the overheating nuclear reactors with seawater in a frantic effort to cool them. The U.S. had already sent two fire trucks to the area to be operated by Japanese firefighters, said Cmdr. Leslie Hull-Ryde, a Pentagon spokeswoman.

Japan’s Chief Cabinet Secretary Yukio Edano says his government expects to ask the U.S. military for additional help. But Lapan said today that no request had been received yet. If a request were received that required troops to go within the 50-mile no-go zone around the power plant, that would be reviewed, he said.

U.S. military assistance so far has focused on delivering relief supplies and helping with logistics and search and rescue missions.

The military is closely watching the nuclear issue and has devised a number of other work-arounds. The Ronald Reagan had to move Sunday out of the downwind path of the reactors after low levels of radiation were detected on the aircraft carrier and among 17 helicopter crew members returning to the deck after a mission.

The USS George Washington also detected radiation while sitting at pier in Yokosuka, and Americans at two bases in the area were advised to limit outdoor activities as much as possible.

And officials said they rerouted an amphibious group to the Sea of Japan on the country’s western coast to avoid the radiation and the debris carried out into the ocean by the tsunami. Sailors and some 2,200 Marines of the 31st Marine Expeditionary Unit are aboard the three-ship group led by the USS Essex, which arrived today, Lapan said.