The situation is improving in Minot, North Dakota, where a trail derailment early Friday morning caused an anhydrous ammonia spill.

About 30 of the Canadian Pacific freight train's 112 cars went off the rails near a subdivision in the city, about 450 km southwest of Winnipeg. It is not known how many of the derailed cars contained the ammonia.

A cloud of ammonia filled the air around the accident site, killing one man and injuring 95 others. Five of the injured are in intensive care.

Randy Schwan, spokesperson for Trinity Hospital in Minot, says he expects the number of patients to grow. Exposure to ammonia can irritate the skin, throat and eyes and cause problems with the pulmonary system and the lungs.

A large area around the site was evacuated after the spill. John Hocking, a brigade chief with the Minot Fire Department, says while the bulk of the spill has been contained, it could be some time before area residents are allowed to return to their homes.

• Cleanup expected to begin Friday night •

CP Rail spokesperson Len Cocolicchio says nobody has been able to get close to the derailment because of the ammonia cloud, but clean-up equipment should arrive at the site late Friday afternoon.

"The environmental contractor is assessing the situation and will determine what we can do safely," he says. "If we get the clearance from the environmental experts and we ensure the anhydrous ammonia is neutralized, we will begin cleanup immediately – that could be as early as this evening."

Cocolicchio says he does not know the condition of the crew on the train. He says they were taken immediately to hospital in Minot.

Schools in the city were closed on Friday and the 37,000 residents of the city have been told to stay indoors.