"One man and one female child are showing no vital signs, said fire department official Yuji Sekizawa, employing a phrase commonly used in Japan to mean the victims have died but the death has not yet been certified by an official medical professional.
Footage broadcast on local TV stations showed multiple police cars, ambulances and fire engines at the scene. Emergency medical tents were put up to treat the wounded.
The fire department said 17 others were injured in the attack, among them more children.
"A man stabbed them," another spokesman for the department, Dai Nagase, earlier told AFP.
"We received an emergency call at 7:44 am, which said four elementary schoolchildren were stabbed."
Police said one suspect, a man, had been detained. NHK said he had stabbed himself, suffering a serious wound. There was no immediate indication of a motive.
The broadcaster said two knives were spotted at the scene, but there was no immediate confirmation from officials.
The attack took place at the time of the morning commute and school run, with one eyewitness saying it occurred by a bus stop.
"I heard the sound of lots of ambulances and I saw a man lying near a bus stop bleeding," the man, who was not identified, told NHK.
"There is another bus stop near the elementary school and I also saw elementary schoolchildren lying on the ground... It's a quiet neighbourhood, it's scary to see this kind of thing happen," he added.