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.
"It is important that visual and chemical monitoring should be intensified to assess the scale of the incident, and potential impact," Johnston added.
The suezmax is lying 150 metres below the surface after a deadly collision and fire 300km east of Shanghai on 6 January. All 32 crew lost their lives.
It was carrying 136,000 tonnes of condensate, plus bunkers.
