Flooding caused by heavy rains have killed 23 people in Iran's north-east Golestan province, the official Irna news agency reported.
At least another 12 people are reported missing after flash floods which hit populated areas during Tuesday night.
Roads, buildings and large areas of farmland have been damaged and rescue teams have been sent to the area.
The effect of flooding has been worsened by intensive deforestation of the mountainous Golestan region.
State television reported that at least 18 people were swept to their deaths in one town alone, Kalaleh, about 500 km (300 miles) north-east of Tehran.
"Cow carcases, refrigerators, combine harvesters and tractors are being swept by," one eyewitness is quoted as saying.
Red Crescent emergency workers quoted by Reuters said the helicopters had been dispatched to look for survivors in the devastated area.