The government said it was his first televised address. The emperor expressed condolences for victims and told the nation not to give up.
"The entire nation is putting forth its best effort to save all suffering people. However, under the severe cold weather, evacuees are having a very difficult time because they lack food, water and energy sources," said Emperor Akihito.
Nearly 8,000 people are still missing after Friday's 9.0-magnitude earthquake and the subsequent tsunami, which was a blast of black seawater that pulverized Japan's northeastern coastline.
The official death toll is at nearly 3,700, but that number is expected to climb to more than 10,000.
Many foreigners have packed the Narita International Airport, trying to get out of Japan.
The emperor also addressed the country's nuclear crisis. Since the quake and wave hit, authorities have been struggling to avert an environmental catastrophe at the Fukushima Dai-ichi nuclear plant. The tsunami knocked out the backup diesel generators needed to keep nuclear fuel cool at the plant's six reactors.
In Japan's tsunami devastated northeast, temporary morgues have been set up in the wrecked remains of public buildings as rescue workers continued to search for survivors.
In Miyagi Prefecture, a school gymnasium has become a temporary resting place for the dead.
Desperate survivors can be seen walking along the lines of stretchers laid out in the mud lifting blankets to try to identify their missing loved ones.
Volunteer firefighter Kenichi Suzuki had been working day and night. He returned home for the first time since the disaster.
"My wife, my son's family and four grandchildren, I lost them all," he said with teary eyes. "I can't take it."
In Iwate Prefecture, several centimeters of snow and a drop in temperatures has further reduced the chances of finding anyone alive.
The Los Angeles County Fire Department's Search and Rescue team continued to assist in the search for victims, though cold weather has made it difficult.
"This morning we woke up with about three to four inches of snow on the ground, which hampered our efforts because of travel on the road," said Batt. Chief Dave Stone. "For 360 degrees, all I could see was destruction, and you just start thinking, 'Where am I going to start with this?'"
Stone said some had up to a 13-minute warning before the tsunami arrived, and that helped save thousands of lives.
The Associated Press contributed to this report.