Several Ukrainian villages are at risk from a giant poisonous cloud that formed after a train carrying a toxic chemical caught fire and derailed.
Hundreds of Ukrainians have been evacuated from their homes after a train carrying highly toxic phosphorous derailed and caught fire near Lviv.
The cloud covers an area of 90sq km (56sq miles) above some 14 villages near the town of Lviv.
At least 20 people were taken to hospital with poisoning after the fire caused a massive toxic cloud.
Hundreds of villagers from the area have been evacuated and at least 20 people have been taken to hospital.
Remaining local residents were told to use gas masks and stay indoors.
The fire aboard the train carrying highly flammable yellow phosphorus was put out late on Monday, reports say.
Ukrainian authorities are trying to establish what caused the crash, but said there was no suggestion of sabotage or terrorism.
The train was travelling from Kazakhstan to Poland and Ukrainian rescue teams are still said to be working at the site of the accident.
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.
Ukraine's Deputy Prime Minister Oleksandr Kuzmuk compared the accident to the blast at the Chernobyl nuclear reactor in 1986.
Fifteen of the train's 58 cars overturned, six of which then caught fire, officials said.
"A disaster has happened. After the Chernobyl catastrophe we are dealing with a case that can pose a real threat for our people," the Associated Press news agency quotes Mr Kuzmuk as saying.
The toxic yellow cloud caused by the blaze covered an area of 90sq km (56sq miles) above 14 villages near Lviv before dispersing on Tuesday.
"It is an extraordinary event, the consequences of which cannot be predicted."
Fire-fighters wearing masks and protective clothing managed to extinguish the fire after several hours.
While hundreds of villagers have been evacuated, those who remain in the area have been advised to stay indoors and avoid eating vegetables or animal produce sourced locally.
'Grave catastrophe'
Phosphorus compounds are mainly used in fertilisers, but can also be used to produce pesticides, cleaning products and explosives.
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.
Authorities advised those remaining not to eat vegetables from their gardens or drink milk produced from their cows.
The Ukrainian Deputy Prime Minister, Oleksandr Kuzmuk, said the toxic cloud was a worrying development and compared the accident to the blast at the Chernobyl nuclear reactor in 1986.
"After Chernobyl we are again confronted with a grave catastrophe which may pose a serious danger to the Ukrainian people," he said.
"The consequences are unpredictable. We are now looking into how dangerous the phosphorus is."
He later backtracked, while other officials were 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.
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