Black smoke was billowing from the East China Sea site where a burning Iranian oil tanker sank, Japanese authorities said Monday, as worries grow about damage to the marine ecosystem from the worst oil ship disaster in decades.

The blazing vessel, which was carrying 136,000 tons — almost 1 million barrels — of condensate, an ultralight, highly flammable crude oil, sank Sunday evening after several explosions weakened the hull.

The tanker Sanchi had been adrift and ablaze after crashing into the freighter CF Crystal on Jan. 6. Strong winds had pushed it away from the Chinese coast, where the incident happened, and into Japan's exclusive economic zone (EEZ).