Iranians are worried by crippling air pollution as "unprecedented" sandstorms mostly originating from neighbouring Iraq hit 20 provinces, forcing the shutdown of schools and government offices.

The blinding sandstorms hit western, central and southern provinces on Wednesday due to winds blowing at high speed, considerably reducing visibility to as low as 50 metres (yards) in some cities.

"Unprecedented sandstorms which entered from west are the most violent storms that have ever reached Iran," said Touraj Hemmati, a top environmental official in the southwestern Khuzestan province which borders Iraq.

Arman newspaper said "small Arabian sands... marked a red situation across the country's sky."

The Kayhan daily said "yellow sand rained in Khuzestan," where the rate of air pollution reached 70 times the permitted amount. It said 123 people were hospitalised in the nearby province of Ilam, also bordering Iraq.

Authorities in six western provinces, including Khuzestan and Ilam, were forced to suspend school and university activities and shut down government offices, Tehran Emrouz daily reported.

Air traffic was partially affected in the country, with incoming flights and departures cancelled in some western cities, according to Hamshahri newspaper.

State air pollution chief Amir Jamali announced on Wednesday that Iran was hit by sandstorms three times more frequently in the first Iranian month of Farvardin, from March 21 to April 20, compared to the same period last year.

"We have witnessed dust storms thrice... Drought that we experienced (last year) has intensified this phenomenon," Fars news agency quoted him as saying.

Iranian media blamed most of the sandstorms on countries west of the Islamic republic, particularly Iraq which is hit by desertification and deforestation due to a decline in the flow of water disrupted by construction of dams as well as a disappearing agriculture industry.

Arman reported that 23 lawmakers, in a letter to President Mahmoud Ahmadinejad on Wednesday, demanded the government "resolve the problems caused by the sandstorms in western and southern provinces."

The weather phenomenon continued Thursday with less intensity in the western provinces, as adverse winds blew further into the central parts of the country. Authorities expect the sandstorms to continue until later in the evening.