HEMEL HEMPSTEAD, (AP) – Explosions at one of Britain”s largest oil depots jolted an area north of London early Sunday, hurling balls of fire skyward, shattering windows and blanketing nearby houses with smoke.

Police said the blasts appeared to be accidental.

Thirty-six people were injured, four seriously, authorities said. Explosions were expected to continue for several hours.

&#34The fire is contained,&#34 said Frank Whiteley, chief constable of Hertfordshire Police. &#34Yes, it”s big and it”s going to burn for some time. But it is under control.&#34

Emergency workers wearing masks cordoned off the area around the Buncefield Oil Terminal, near the town of Hemel Hempstead. Authorities evacuated residents as the plume moved slowly eastward.

Whiteley cautioned that the cloud contained irritants that could cause coughs and nausea. People in the path of the smoke should stay indoors, he said.

There was nothing to indicate the blasts were acts of terrorism, police said, although the al-Qaeda terror network and other terrorist groups have threatened to target fuel deposits. Britain has been on edge since the July 7 London transit bombings, which killed 56 people including four suicide bombers, and a failed attack on July 21.

Some residents reported hearing an aircraft flying low overhead shortly before the first explosion at around 6 a.m. (0600 GMT) but police said there was no evidence of a plane crash.

Nearby Luton Airport remained open and flights were operating normally.

&#34All indications at this stage are that this was an accident,&#34 Whiteley said. &#34However, clearly we will keep an open mind, as with all investigations, until we can confirm that for certain.&#34

The Ramada Hotel in Hemel Hempstead, about 2 miles (3 kilometers) from the oil terminal, was evacuated after windows were shattered by the blast. Two guests suffered minor injuries from broken glass and were treated at the hotel, said Calum Russell, marketing director for Jarvis Hotels Ltd. All 187 guests had since returned to their rooms, he said.

Residents in Hemel Hempstead reported a loud boom and some felt their houses shake. The blast was so large it was felt throughout a large part of southeast England including London, 25 miles (40 kilometers) away.

&#34Around 6 a.m., as we were sleeping, there was a mighty explosion, a thunderclap that woke me up,&#34 said Neil Spencer, 42, who lives about three-quarters of a mile (1 1/2 kilometers) from the terminal. &#34It was fireball after fireball, truly amazing.&#34

The area around the fuel depot was being evacuated. Police said many roads and highways had been closed.

&#34There was a loud boom and the house shook violently,&#34 said Duncan Milligan of Hemel Hempstead, who said the blast woke him up. &#34I am about 3 miles from where the explosion took place but I can see flames high in the sky and smoke billowing everywhere. There is clearly a building on fire near the motorway and police and emergency services are everywhere.&#34

Resident Graeme North reported a &#34great column of smoke&#34 at the oil depot and said the blast damaged several houses nearby. &#34The garage door was blown open,&#34 he told Sky television. &#34The blast wave blew bath panels off.&#34

Local resident Richard Ayers said a massive column of smoke rose into the air and said the force of explosions had blown the roofs off houses near the oil depot. &#34It is like it is doomsday,&#34 he told the British Broadcasting Corp.

Britain”s deadliest oil-related disaster was the July 6, 1988, explosion and fire on the North Sea oil platform Piper Alpha off the Scottish coast, which killed 167 workers.