UPDATE: The NSW Ambulance Service says paramedics assessed eight people after the Singleton truck crash. Four were taken to hospital, two by road and two via helicopter.
One patient, a 67 year old man is in a critical but stable condition.
"It was quite a distressing scene for all those who have attended," Inspector Luke Wiseman said.
UPDATE: Police have launched a critical incident investigation following this morning's truck crash.
They say road spikes were used in an attempt to stop the stolen semi-trailer once it reached Singleton.
The driver has been arrested and is being questioned.
A 67 year old man who was in one of the cars hit by the truck, has been flown to John Hunter Hospital in a critical condition.
EARLIER: An emergency situation is unfolding in Singleton where a semi-trailer has burst into flames after hitting several vehicles and the Royal Hotel.
George Street is closed as firefighters rush to the scene near Elizabeth Street.
The truck had been stolen from a servo at Murrurundi, while the driver was paying for fuel.
Paramedics are treating four patients. Three are being taken to hospital. NSW Ambulance says two have "significant" injuries.
NSW Fire and Rescue says the truck was carrying fertilizer.
The New England Highway is closed in both directions between Ravensworth and Whittingham
Diversions are in place via Lemington Road and the Golden Highway.