Four people were killed and seven were injured when a massive crane collapsed at LyondellBasell's Houston refinery on Friday afternoon, a refinery executive said.

"It's a very sad day for us at the refinery," said Jim Roecker, LyondellBasell's vice president of refining operations, during a news conference outside the refinery's front gate.

All workers at the refinery have been accounted for, Roecker said.

Production at the 270,000 barrel per day (bpd) refinery was not affected by the crane collapse, said refinery spokesman David Roznowski.

The crane was being readied to work in the overhaul of a coking unit at the 270,000 barrel per day refinery when it

collapsed at about 1:30 p.m. local time (7:30 p.m. British time), the company said.

In addition to the coking unit, a crude distillation unit had been shut at the refinery for a planned seven-week overhaul. The refinery has two coking units and two crude units.

Roznowski declined to discuss the refinery's current level of production. He said it would be at least several days before Lyondell could determine how the crane collapse would affect the planned overhaul.

Roecker said the crane was operating, but was not lifting anything.

"It certainly is too early to tell what caused the issue," he said.

The crane fell on a tent which KTRK-TV in Houston said was used for worker meals.

The television station interviewed a man at a Houston hospital who was identified as one of the injured workers.

"All I saw was our lunch tent was smashed," Michael Gabriel told KTRK.

Roznowski said he could not confirm if the tent was used for meals, breaks or to stage equipment being used in the overhaul.

The crane collapse in Houston was the deadliest U.S. crude oil refinery accident since a 2005 explosion at BP's giant refinery in Texas City, Texas, killed 15 workers and injured 180 other people. Texas City is located 50 miles south of Houston.

Following a two-year investigation of the BP blast, the federal Chemical Safety Board recommended refiners not place workers in structures near process units incapable of withstanding catastrophic explosions.

All of the workers killed at BP Texas City were in temporary work trailers near the site of the explosion.

The fatalities in Houston follow two recent deadly crane collapses in New York. In May, two people were killed on Manhattan's Upper East Side after a large crane fell and damaged an apartment building. In March, seven were killed after a crane crushed a residential building.

Roecker declined to disclose the names of those killed and the companies for which they worked, pending notification of the next of kin.

He said the crane was owned by Deep South Crane & Rigging, which is based in Baton Rouge, Louisiana.

Deep South said in a statement it had few details about the accident.

"Our thoughts and prayers are with our employees and their loved ones," the company said.

Deep South is providing crane equipment to a number of large U.S. refinery projects, including expansions at the Motiva refinery in Port Arthur, Texas; the Marathon refinery at Garyville, Louisiana; and maintenance projects at Citgo's Corpus Christi, Texas, refinery and the ExxonMobil refinery in Joliet, Illinois.

Those killed and those injured worked for outside contractors preparing a seven-week overhaul of one of two coking units and one of two crude distillation units at the refinery located along the Houston Ship Channel, Lyondell said. The work began in early July.

A crude distillation unit does the initial refining of crude oil and a coker strains the last refinable material from crude oil.

LyondellBasell is a Netherlands-based chemical and refining company with annual revenues of $45 billion and 16,000 employees worldwide.