TOKYO — Workers made incremental progress at the stricken Fukushima Daiichi nuclear plant on Thursday, but disturbingly high radiation readings there as well as miles away continued to reinforce fears that Japan’s crisis was far from over.

The death toll from the earthquake and tsunami that crippled the plant three weeks ago rose to nearly 11,600, with more than 16,000 people listed as missing. Hundreds of thousands of people are still homeless, including tens of thousands who have been displaced from the area around the nuclear plant. Workers have been dousing the reactors and spent fuel pools at the Fukushima Daiichi plant with water to prevent meltdowns and frantically trying to restore power and restart the cooling systems, but the resulting floods of dangerously contaminated water have complicated the efforts.

Workers prepared more tanks on Thursday to transfer radioactive water from the turbine buildings at Reactor Nos. 1, 2 and 3 to keep it from flowing into the ocean. But readings taken in the sea near the plant showed that levels of the radioactive isotope iodine 131 have continued to rise, testing at 4,385 times the statutory limit on Thursday, nearly four times higher than on Sunday, said Hidehiko Nishiyama, deputy director general of Japan’s Nuclear and Industrial Safety Agency. That rise increases the likelihood that contaminants from the plant are continuously leaking into the sea, he said.

Iodine 131 was also detected at levels 10,000 times the safety limit in groundwater near Reactor No. 1. However, the government asked for retesting after the plant’s operator, the Tokyo Electric Power Company, which is known as Tepco, cast doubt on its own data not long after divulging the initial figures.