Dozens of people have died and at least 50 are missing after torrential rain triggered landslides and flash flooding in western Japan on Saturday.
Local authorities said 20 people had been killed, while public broadcaster NHK said the death toll had risen to 66.
The number of casualties is expected to rise, said Yoshinobu Katsuura, a disaster management official in Ehime prefecture.
The prime minister, Shinzo Abe, said the situation was "extremely serious" and ordered ministers to "make an all-out effort" to rescue victims.
The rainfall has hampered rescue operations in Hiroshima, Ehime, Okayama, Kyoto and other regions.
Water levels reached five metres (16ft) in the worst-hit areas, forcing some residents on to rooftops and balconies to attract the attention of rescue helicopters.
Almost 2 million people, mainly in western Japan, have been told to evacuate their homes, according to the fire and disaster management agency, but NHK said the figure had risen to nearly 3.2 million. Some areas have been hit by more than one metre of rain, while almost 48,000 soldiers, police and firefighters have been deployed for rescue operations, said the chief cabinet secretary, Yoshihide Suga.
Another 21,000 troops are on standby.
The Japan Meteorological Agency upgraded its alert system to the highest level - only issued when the amount of rain is expected to be the highest in decades - in large areas of western Japan while lifting the warning in other regions.
Among the missing were five people buried when houses collapsed in Hiroshima prefecture.
In Ehime prefecture a woman was found dead on the second floor of a home hit by a landslide, Kyodo said.
Yamaguchi prefecture, another area hit by the heavy rain, alerted people to heed evacuation warnings and act quickly.
Kyoto prefecture said it was working to control flooding at several dams.
About 250 people had to flee their homes and the prefecture identified one fatality as a 52-year-old woman.
Roads were blocked in some areas and warnings were issued on landslides.
Military water trucks were rushing to areas where water systems were no longer working, Okayama prefecture said.
Although Japan is among the most modernised of Asian nations, rural areas are hit hard by the rainy season each year, often resulting in casualties and heavy damage.