LONDON, Dec. 11 (Xinhuanet) -- British police said on Sunday that they believed there were around 36 casualties in the oil depot blasts near Hemel Hempstead, around 40 km north of London, of which four were seriously injured.

    "All indications are that the incident was an accident," police said at a news conference.

    The fire was under control, police said, adding further explosions are expected in the morning.

    "Plume of smoke is an irritant and we advise the people to keep away. We advise the public to stay indoors and close windows," said police.

    Police also appealed the people not to panic to buy petrol.

    The huge blast, followed by several smaller ones, appeared to be centered around the Buncefield fuel depot near junction 8 of the M1 motorway at 6:03 am local time (0603 GMT), and was so powerful that could be felt more than 30 km away.

    Houses near the Buncefield fuel depot have been evacuated and emergency services are handing out gas masks.

    The cloud of acrid smoke is moving south and beginning to obscure main roads, including the M40 motorway, according to reports.

    Experts said all the fire crews can do in this type of situation is to cut off the fuel supply and wait for the burning out of the fire, during which there is "every chance of another explosion." Enditem