21 tankers of low-quality fuel have been sent back to Iran, says the Afghanistan National Standards Authority (ANSA).
Nine tankers of substandard diesel and petrol were returned the Islam Qala port of Herat province and 12 from Farah province.
The state-run Bakhtar News Agency reported the imported fuel did not meet the required standards and was sent back to the neighboring country.
In an attempt to block low-quality fuel imports into the country, ANSA has returned hundreds of tankers to Iran in recent months.
In a statement, the authority ANSA asked traders to import quality goods into the country. Importers violating quality standards would face legal action, it warned.