Four workers were killed and two others are in hospital today after one of the world's largest cranes collapsed at a U.S. oil refinery.

Five other workers were also hurt when the 300ft crane - capable of lifting a massive 500 tons - toppled in Houston, Texas.

The crane fell on top of a smaller crane and a workers' rest tent, but officials at the 700-acre refinery are still unsure whether the casualties had been on the larger crane or underneath it when it collapsed.

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The giant red crane lies across the wreckage of a smaller yellow one at the Texan refinery

The names of those killed have not been released.

Two severely injured survivors were flown to hospital by helicopter.

'This is a traumatic experience for all of us,' said Jim Roecker, a vice-president at U.S.-Dutch refinery owner LyondellBasell, one of the world's largest chemical companies.

All four dead were employees of the crane's owner Deep South, based in Baton Rouge, Louisiana.

'Our thoughts and prayers are with our employees and their loved ones,' the company said in a statement.

It was investigating 'to determine the root cause, correct it and ensure that this type of tragedy does not occur again', the statement added.

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The crane appears bottom left against the backdrop of the refinery and the Houston Ship Channel

The crane was not lifting anything when it fell  at about 1.30pm local time yesterday (6.30pm GMT).

The gigantic mobile crane, which has a 400ft-long boom, had been set up at the refinery to lift a giant steel derrick.

It was part of a seven-week overhaul of an on-site coking unit, a device which squeezes the last refinable material from a barrel of crude oil.

Oil company Lyondell had taken the unusual step of planning an overhaul in July, when most US refineries run at high capacity to meet peak summer demand.

'There are only a few cranes of this size in the world,' a spokesman said. 'You do the work when the crane's available. It was all about the availability of the crane.'

The fatalities in Houston follow two crane collapses in New York in March and May that killed a total of nine people.

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Devastation: Cables and crushed metal strew the ground under the toppled crane

Neither Houston nor Texas as a whole perform safety inspections of giant construction cranes, according to the Houston Chronicle - while Texas is one of U.S. 35 states that do not require crane operators to be licensed.

Production at the Lyondell refinery - running at 270,000 barrels per day - has continued as usual despite the accident.