TOKYO — Japanese officials struggled on Sunday to contain a widening nuclear crisis in the aftermath of a devastating earthquake and tsunami, saying they presumed that partial meltdowns had occurred at two crippled reactors and that they were facing serious cooling problems at three more.

The emergency appeared to be the worst involving a nuclear plant since the Chernobyl disaster 25 years ago. The developments at two separate nuclear plants prompted the evacuation of more than 200,000 people. Japanese officials said they had also ordered up the largest mobilization of their Self-Defense Forces since World War II to assist in the relief effort.

On Saturday, Japanese officials took the extraordinary step of flooding the crippled No. 1 reactor at Fukushima Daiichi Nuclear Power Station, 170 miles north of Tokyo, with seawater in a last-ditch effort to avoid a nuclear meltdown.

Then on Sunday, cooling failed at a second reactor — No. 3 — and core melting was presumed at both, said the top government spokesman, Chief Cabinet Secretary Yukio Edano. Cooling had failed at three reactors at a nuclear complex nearby, Fukushima Daini, although he said conditions there were considered less dire for now.