Environmental group Greenpeace has said the slick from the Sanchi could have reached Japanese islands.
It said oil from the sunken NITC tanker may have been detected at Takarajima.
"It appears that the island of Takarajma is in one of the higher risk areas predicted by the National Oceanography Centre," said Paul Johnston, a Greenpeace International science unit official, according to the DPA news agency.
