Rescue work continues in Western Ukraine as emergency services try to clean up a chemical spill. On Tuesday, a train carrying yellow phosphorus derailed and burst into flames near the city of Lvov, releasing toxic gas into the air.

Toxic fumes have contaminated an area of 90 square kilometres. Hundreds of homes in nearby villages had to be evacuated, and a number of people fled on their own.

Around 70 people are reported to have sought medical help, 19 of them children. 25 of them are rescue staff who were working at the scene. Doctors say there's no threat to their lives.

Firefighters were delayed because burning yellow phosphorous cannot be put out with water.

“A disaster has taken place in our country. After the Chernobyl catastrophe we are dealing with a serious situation that could be dangerous for the citizens of Ukraine. This is an emergency situation. It's almost impossible to forecast its consequences,” Aleksandr Kuzmuk, Ukraine's Vice-Premier, said.

Officials have not said what caused the incident, although the transport ministry says faulty train cars could have been to blame.

Phosphorus is a chemical element known for its ability to ignite upon exposure to oxygen. Yellow phosphorus is a highly toxic variety of this element. It is traditionally used in fertilizers, fridges, the metal industry and weapons. If ingested or inhaled, it can cause nausea and heavy burns. Long term exposure could lead to more serious illnesses and even death.