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More than two dozen children have been rushed to hospital after a carbon monoxide leak at a childcare centre in the US state of Pennsylvania.

Every ambulance in the city of Allentown was deployed to the Happy Smiles Learning Center on Tuesday.

The early morning call prompted a large emergency response as multiple people were showing symptoms of poisoning.

Some 25 children and eight staff members were taken to hospital, with the majority now in a stable condition.

Carbon monoxide poisoning can result in severe sickness and even death.

"It was initially a medical call for a child who was unconscious," Captain John Christopher of the Allentown Fire Department told the ´óÏó´«Ã½.

But firefighters and paramedics who arrived at the scene found the monitors on their bags indicated high levels of carbon monoxide.

"This prompted immediate evacuation by roping in all the city ambulances," Mr Christopher said.

The children and staff were taken to all four of the city's hospitals after initial medical aid was provided at the scene.

The childcare centre is an isolated building so there is no risk of the leak spreading further, the fire official said.

Nine-year-old pupil Danyelis Polanco said she quickly knew something was wrong after entering her school building and walking towards her classroom.

"My nose was bothering, my eyes were burning bad," she told CBS, the ´óÏó´«Ã½'s US partner.

"I felt a little dizzy. My head was hurting a little," she continued.

"And when I got there I was a little worried because I saw someone from my class who was on the floor and I was worried because his eyes were closed and stuff."

Local newspaper The Morning Call quoted a gas company official as saying the leak was caused by a malfunctioning heating unit and a blocked venting system, but this has not been confirmed by the authorities.

Carbon monoxide is an odourless, colourless gas produced by the incomplete burning of carbon-based fuels such as oil, wood and coal. The gas is harmful because it displaces oxygen from red blood cells, resulting in damage to major organs.

More than 400 people in the US are killed and 4,000 are admitted to hospital every year when they unintentionally inhale too much carbon monoxide, according to statistics provided by US Centers for Disease Control and Prevention.