Hundreds of Ukrainians have been evacuated from their homes after a train carrying highly toxic phosphorous derailed and caught fire near Lviv.
At least 20 people were taken to hospital with poisoning after the fire caused a massive toxic cloud.
Remaining local residents were told to use gas masks and stay indoors.
Ukrainian authorities are trying to establish what caused the crash, but said there was no suggestion of sabotage or terrorism.
The freight train, which was en route from Kazakhstan to Poland, derailed near Lviv, a town not far from the Polish border in western Ukraine on Monday night.
Fifteen of the train's 58 cars overturned, six of which then caught fire, officials said.
The toxic yellow cloud caused by the blaze covered an area of 90 sq km (34.7 sq miles) above 14 villages near Lviv before dispersing on Tuesday.
Fire-fighters wearing masks and protective clothing managed to extinguish the fire after several hours.
Phosphorus compounds are mainly used in fertilisers, but can also be used to produce pesticides, cleaning products and explosives.
The chemical can cause damage to the liver, the heart or the kidneys if consumed. In many cases exposure can be fatal.
Many people were evacuated at their own request.
Deputy Prime Minister Olexander Kuzmuk, who on Tuesday compared the spill to the 1986 Chernobyl nuclear disaster, has since insisted there is no health risk to surrounding villages, the AFP news agency reported.
Other officials have been careful to play down any comparisons with Chernobyl and said they had minimised the risk to public health.
"The cloud of a toxic gas dispersed and there is no threat to people's lives," Ihor Krol, a spokesman for the Emergency Situations Ministry, told the Associated Press.