A passenger bus crash has killed three people and and injured 14 others in southern Illinois.

The St-Louis bound Greyhound bus crashed into three tractor-trailers parked along a highway rest area exit early on Wednesday, state police said. 

Four people were taken to the hospital by helicopter and at least 10 others were taken by ambulance, they said. 

State Police spokesperson Melaney Arnold said those killed and injured in the Madison County accident were all on the bus.

Ms Arnold was not sure if the driver was among those killed or injured. 

No one in the three trucks was hurt in the crash.

Passenger Edward Alexander said he helped a pregnant woman get off the bus and was searching for his phone when he “realised smoke was coming in the bus”.

“I was like, ‘forget that phone’, and went on and jumped out the window,” he told the St. Louis Post-Dispatch.

Greyhound spokesperson Mike Ogulnick said the bus was carrying about 30 people travelling from Indianapolis to St. Louis.

“Our primary concern is ensuring we care for our passengers and driver at this time,” Mr Ogulnick said.”

We are working closely with local authorities, and a relief bus is on the way for passengers.”

It is illegal in Illinois for trucks to park on exit ramps.

Trucking industry experts say semi-trailers often stop there for the night because overnight parking is hard to find at rest areas and other places such as truck stops.

“Rest area safety is one of the issues that will be a part of this investigation,” National Transportation Safety Board Tom Chapman said. 

AP