Russian oil product cargoes are building up on tankers with no confirmed destination due to caution among South Korean buyers.
Analytics firm Kpler said more than 2m barrels of naphtha have been held on vessels for more than a week.
Many of these tankers remain in waters near Oman, it added.
Naphtha is a key component in plastics manufacturing.
The average amount on tankers was 790,000 barrels each week in January and February, the firm said.
South Korean petrochemical companies are usually major buyers of Russian oil.
But they are now steering clear of imports, according to traders cited by Bloomberg.
This is the result of an investigation into naphtha shipments launched in March by South Korean authorities.
The probe aims to find out if cargoes from Russia were being re-labelled.
Following the war in Ukraine, Russia began to find alternative markets for naphtha in the United Arab Emirates, Malaysia, Singapore and Tunisia, Kpler noted.
South Korea has also been sourcing more products from Kuwait and Oman, with increased amounts of Russian barrels also going to China and Taiwan.
Importers in South Korea have to comply with the G7 price cap on Russian oil.
