SAN FRANCISCO (AFP, NYTIMES, DPA) - Four people were killed and four others were injured when a crane collapsed on Saturday (April 27) afternoon in the north-western United States city of Seattle, the fire department said.

In a statement, the fire department said the crane "fell from the roof of a building" and hit six vehicles below, near the southern shore of Lake Union, which lies in the centre of the city.

It said three men and one woman were killed - two of them were operators in the crane, while the others were in different cars.

Seattle's fire chief Harold Scoggins said in a news conference did not say if the cars were moving or stopped in the street at the time of the collapse.

Of the four people injured, a 27-year-old man, a 25-year-old woman and a four-month-old baby girl were taken to hospital with non-life threatening injuries, the department said.

The Seattle Times reported that the crane was used as part of construction work to build a new Google campus in the city, a fast-growing tech hub famously home to e-commerce giant Amazon.

"It was terrifying," Ms Esther Nelson, a biotech research assistant who was working in a building nearby, told the paper. "I looked up. The wind was blowing really strong," she recalled.

Ms Nelson said she observed what happened from a break-room window. She said she saw the crane snap in half and saw one half collapse onto the building and the other half plunge to the street.

It was not immediately clear what caused the crane to fall or whether construction was going on at the time of the collapse.

Some said on social media that they had observed strong winds in the area of the collapse, but the National Weather Service said on Twitter that wind measurements nearby at the time were not considered as strong wind gusts.

Images shared on social media captured the dramatic aftermath of the crane's collapse. At least one part of the yellow crane could be seen in the street as emergency personnel gathered. Other images showed another piece of the crane on top of the building.

"A full and thorough investigation into the cause of the crane failure is being conducted by Washington State Department of Labour and Industries," the fire department said.

Once home to the city's logging and maritime industries, the South Lake Union neighbourhood, where the crane fell, has grown substantially in the last 15 years, like Seattle as a whole.

Many technology companies and research laboratories are there, along with restaurants and residential buildings.

Amazon and the Seattle Cancer Care Alliance have offices in the neighbourhood, according to the South Lake Union Chamber of Commerce.