Iran warned Sunday of a resurgence of the COVID-19 pandemic as it registered 51 new deaths.

"The situation should in no way be considered normal" in Iran, health ministry spokesman Kianoush Jahanpour said in televised remarks on Sunday.

"This virus will be present" for the time being, he added.

Authorities reimposed more stringent measures in the southwestern Khuzestan province, reversing a phased return to work meant to revitalise the battered economy.

The new fatalities raised the overall confirmed death toll to 6,640 since the country reported its first cases in February 19.

Iran has allowed a phased return to work since April 11 and has since also reopened mosques in parts of the country deemed to be at low risk.

But Jahanpour said Iran was "witnessing a critical situation in Khuzestan province and to an extent in Tehran," AFP reported.

He also said that 1,383 new COVID-19 cases were confirmed in the past 24 hours, raising total infections to 107,603.

Out of those hospitalized, 86,143 had recovered and were discharged, but 2,675 were in critical condition.

Experts and officials both in Iran and abroad have cast doubt over the country's COVID-19 figures, saying the actual number of cases could be much higher.

Both the capital Tehran and Khuzestan remained at "red", the top level of its color-coded risk scale.

In the capital, a member of the virus taskforce warned that current health protocols could not contain the spread of the illness in Tehran.

"With businesses reopening, people have forgotten about the protocols," Ali Maher told ISNA news agency.

"Maybe it was too soon" for a return to normal life, Maher said.

President Hassan Rouhani meanwhile said in a televised meeting that schools would partially reopen next Saturday.

This applied only "for students seeking to meet and talk to their teachers" and attendance would not be mandatory, he said.

Cinemas, stadiums and universities remain closed across Iran.