They said at least 170 people had died in the floods and hopes were fading for another 160 still missing.
Army and Red Crescent helicopters scoured swathes of flooded farmland for the fourth day on Monday looking for survivors.
"There is a high possibility that most of the missing may be dead, but because of the bad weather, we cannot find them," the IRNA news agency quoted Habibzadeh Dabagh, a local official in the afflicted area, as saying.
"Based on the reports of the rescue teams, the death toll will definitely rise."
Rescue teams have so far picked up some 7,500 villagers, but some 800 others are still stranded in the worst affected areas in Golestan and Khorasan provinces, state radio said.
The flash floods hit on Friday after unseasonal heavy rains began to lash the area. Some 15,000 hectares (37,000 acres) of farmland is now under water and about 1,500 houses have been destroyed.
Television showed pictures of buses and cars overturned by the floods and survivors perched on homes and small islands of dry land awaiting rescue.
Bridges and roads were swept away in muddy torrents as rivers burst their banks.
Gas supplies, drinking water and roads, including the main link to neighbouring Turkmenistan have been cut off. Damage is estimated to run into tens of millions of dollars.
Local officials have called for volunteers to help Red Crescent aid workers search for the missing. A bank account has been opened for donations to help the flood victims.
Many newspapers criticised what they said was the government's lack of preparedness to deal with the disaster.
"Much of the loss of life and destruction of property could have been avoided or at least lessened, if the government was effectively prepared and existing strategies to counter natural disasters had been implemented," the Norouz newspaper quoted university lecturer Hamid Niyazi as saying.
- Reuters - Thomson Reuters Foundation
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