Six people have now died from the toxic waste dumped in the biggest Ivory Coast city, Abidjan, while 9,000 have sought treatment, the government says.
The dumping provoked fury among Abidjan residents
The UN has set up an inter-agency team to respond to the crisis.
A French team is analysing the waste, and says it will produce a report by the end of this week.
The Ivorian government resigned after protests over the toxic waste, and no new administration has yet been formed, threatening the fragile peace process.
The waste came from the ship, the Probo Koala, and has been found in some 10 open-air sites around Abidjan.
Residents complain of a smell of rotten eggs on the streets. Environmental group Greenpeace say the sludge dumped is made up of some 400 tonnes of oil-refining waste.
The company which owns the ship, Trafigura Beheer BV, says it is extremely concerned and has sent oil and mining experts to Abidjan to help the authorities.
It says it informed the authorities and handed over discharges from washing out its oil tanks with caustic soda - known as slops - to a certified local company, Compagnie Tommy.
A UN agency said that many of the sites where dumping occurred have still to be fenced off.
"Most of them don't have signalling or warning," it said.
Three company heads are reported to be among seven people arrested in the investigations.
Those affected are being treated for stomach problems, nosebleeds and respiratory illnesses.
The hospital has set up a specialist unit to deal with those suffering from similar symptoms.