The Sanchi is still alight and bad weather - with waves of up to 4m (13ft) - is hampering the rescue work.
The Sanchi tanker has on board 136,000 tonnes of condensate, which is an ultra-light version of crude oil.
The Panama-flagged Sanchi was bringing condensate from Iran to South Korea when the collision with the Hong Kong-registered freighter CF Crystal, carrying grain from the US, happened in the East China Sea.
The tanker, run by Iran's leading oil shipping operator, has on board about one million barrels of condensate, which at current prices is worth roughly $60m (£44m).
The Atlantic Empress incident listed below - the record spill from ships - saw little oil reach coastlines, whereas the Exxon Valdez, which spilled eight times less, is considered one of the world's worst environmental disasters.