A United Nations expert says he has found strong evidence linking at least 15 deaths and thousands of sick people to waste dumped by a ship in Ivory Coast in 2006.
Okechukwu Ibeanu's report into the incident will be presented to the UN Human Rights Council in Geneva this week.
But Trafigura, the international commodities trader that chartered the ship, repeatedly denies any wrongdoing and says the UN report is "deeply flawed".
On the evening of August 19, 2006, the Probo Koala ship, chartered by Trafigura, offloaded 500 tons of "slops" – petrochemical waste, described as residues from gasoline mixed with caustic washings – in the capital Abidjan to a local company, which dumped it in 18 open-air city waste tips. The firm had previously attempted to offload the cargo in Amsterdam.
The report said thousands of local residents visited health centres complaining of nausea, headaches, vomiting, abdominal pains, skin, throat, lung and gastric problems the next day.
Fifteen people died, 69 were hospitalised and there were more than 108,000 medical consultations resulting from the incident, it added.
For Okechukwu Ibeanu, the UN Special Rapporteur for toxic waste, there are clear links between the dumping of the waste and the deaths and illnesses.
"It could not have been a coincidence that thousands of people in the immediate aftermath of this event showed consistent symptoms. It is difficult not to conclude that there was a connection," he told journalists in Geneva on Thursday.
"The point to be made is that there is still some contention among scientists as to the actual causality," he admitted.
Trafigura said it had cooperated fully ahead of Ibeanu's report, which it described as "deeply flawed" and containing "potentially damaging conclusions that are entirely unsupported by verifiable evidence".
In a statement on Wednesday the multinational, which has bases in London, Amsterdam and Geneva, said it "has always denied and continues to deny any liability for events that occurred in the Ivory Coast".
It repeated that the waste it offloaded "could not possibly have caused deaths and serious or long-term injuries".
The company claims it sought at all times to comply with all relevant regulations and procedures concerning the offloading of the slops in Abidjan.
But it couldn't have foreseen the "reprehensible and illegal way" in which Compagnie Tommy [the local subcontractor] then proceeded to dump the slops, it claimed.
Trafigura said it had appointed an independent investigation by WSP Environment and Energy earlier in 2009, which failed to identify any compounds specifically related to the slops at tested sites. WSP concluded there was no related risk to human health caused by the slops at these sites, it declared.
However, environmental group Greenpeace said on Thursday it had obtained internal emails and other documents that show Trafigura managers were aware the waste that the Probo Koala brought to the Ivory Coast was hazardous.
Trafigura managers "knew they were violating the rules by exporting toxic waste from Europe, and it was deliberately taken to Africa to dispose of there because it was cheaper", Greenpeace spokesman Andre van der Vlugt told Associated Press.
Van der Vlugt refused to say how his organisation obtained the documents or verified their authenticity. They have been published on the website of Britain's Guardian newspaper.
Civil and criminal proceedings are ongoing in both Britain and the Netherlands on issues including the chemical composition of the waste, responsibility and compensation, Ibeanu said.
Trafigura says it is close to offering compensation to 31,000 West Africans who claim they were harmed by hazardous waste dumped around the Ivorian capital. The million-dollar class action was scheduled to be heard in an English court in London next month.
"It currently appears that this settlement is likely to be acceptable to most, if not all, of the claimants," Trafigura and British law firm Leigh Day & Co, representing the claimants, said on Wednesday.
But an Ivorian group said the compensation offer did not go far enough, adding that a long-term resolution was needed because there was still waste in Abidjan that had not been removed and there were zones that still need cleaning up.
In 2007 Trafigura paid $198 million (SFr204 million) in damages to the victims of the toxic poisoning in Ivory Coast in an out-of-court deal with the Ivorian government which exempts it from legal proceedings in that country.
Simon Bradley in Geneva, swissinfo.ch with agencies