Four people were seriously injured today when a series of huge explosions ripped through an oil terminal.

They were among 36 hurt in the blasts, which rocked the Buncefield depot near Hemel Hempstead in Hertfordshire, police said.

The force of the blasts could be heard up to 40 miles away as flames shot more than 200 feet into the sky.

Hertfordshire police chief constable Frank Whiteley said detectives are treating the incident as an accident.

The first blast happened just after 6am at the fuel terminal in Leverstock Green, Herts, close to Junction 8 of the M1.

The depot - which holds huge stocks of various fuels - also supplies aviation fuel for Heathrow and Luton airports.

Mr Whiteley said 100 police officers are at the scene alongside more than 100 firefighters. He said it was the largest fire he'd ever seen.

More explosions are expected at the scene but Mr Whiteley stressed that the fire is under control and should not spread to a larger area.

The public are being advised to stay indoors and not go near the immediate area of the blasts. Anyone concerned about the blasts should ring the Casualty Bureau on 0800-096 095. Those calling from overseas should call 020-7158 0125.

If anyone is feeling unwell they are advised to call NHS Direct on 0845 4647.

Fire crews are putting plans in place for several days of work.

Mr Whiteley said: "All indications at this stage are that this was an accident. However, clearly we will keep an open mind until we can confirm that for certain."

As a result of operations at the scene, it was anticipated that there would be further explosions today, and he wanted to reassure the public that this was nothing untoward.

He said it was believed there were around 36 casualties, of whom four were more serious.

A plume of smoke above the site was moving slowly eastwards.

"This cloud, not least because it does contain heavy smoke, is an irritant, and would certainly make people who inhale it potentially cough, potentially irritate the eyes, and potentially feel nauseous.

"Our strong advice is to keep out of the plume of smoke, and if the windows of your house have been blown out or are damaged, we would suggest you should evacuate yourself somewhere safer.

"People should stay indoors, close your windows, and watch the news for updates."

Howard Borkett-Jones, medical director of the West Hertfordshire Hospitals NHS Trust, said that a total of 36 people had been treated at the Hemel Hempstead general hospital for injuries from the explosions.

Of those, he said 34 had minor cuts and bruises and all had been treated and discharged from the Accident and Emergency department.

One patient had been transferred to Watford general hospital for "respiratory support" and one had been admitted to the Hemel Hempstead General and was under observation.

He said the hospital was running effectively as normal but that extra capacity was available to cope with any "escalation".

"We have thoroughly prepared plans for major incidents of this nature and we regularly test them out."