Iranian authorities ordered scores of villages to be evacuated on Tuesday as the effects of severe flooding spread further across the country.
Meanwhile, Washington denied claims by Tehran that US sanctions were slowing aid work.
At least 47 people were killed in the past two weeks by flash floods after the worst rain in Iran for at least a decade.
The armed forces had intensified their relief work, with military and Red Crescent helicopters taking part in rescue operations.
Flood risks forced authorities to evacuate more than 70 villages in the oil-rich south-western province of Khuzestan, state news agency Irna said.
Iran's supreme leader, Ayatollah Ali Khamenei, praised rescuers but said officials should have better anticipated the disaster, state TV reported.
Iranian Foreign Minister Javad Zarif said on Monday that US sanctions, reimposed after Washington quit a 2015 nuclear deal between Iran and six world powers, were impeding aid efforts for affected communities.
"Blocked equipment includes relief choppers. This isn't just economic warfare, it's economic terrorism," Mr Zarif said on Twitter
In a response that Mr Zarif described as fake news, US Secretary of State Mike Pompeo said Washington was ready to help through the Red Cross and Red Crescent.
Mr Pompeo accused Iran's clerical establishment of "mismanagement in urban planning and emergency preparedness".
"The regime blames outside entities when, in fact, it is their mismanagement that has led to this disaster," he said.
Germany and Britain, also signatories to the nuclear deal, offered humanitarian aid including 40 boats, Irna reported.
Flooding has affected at least 26 of Iran’s 31 provinces since heavy downpours began on March 19.
A state of emergency was declared in several provinces and tens of thousands of people have been moved to safety.
Electricity and telecommunications were cut in affected areas, roads had been washed away and people were waiting on rooftops to be rescued in some villages.
President Hassan Rouhani, accused by critics of mishandling the crisis, has promised to compensate all of those affected.
Authorities are concerned about increasing threats of dam failures that could wreak more damage.
Tehran was on alert for flooding and officials urged people to stay away from vulnerable areas.