Fire destroyed a seven-storey apartment building under construction in downtown Los Angeles and spread to a neighbouring high-rise before flames were largely extinguished, forcing a major freeway shutdown through rush hour.

The blaze erupted at about 1:30am local time on Monday and took about three hours to bring under control, but no injuries were reported, said Katherine Main, a spokeswoman for Los Angeles City Fire Department.

About 250 firefighters, roughly a quarter of the city's total on-duty force, were battling the flames at the height of the blaze, she said.

Authorities believe the construction site was empty when the fire was sparked. Photo: Damian Dovarganes/AP

The cause of the fire, and a separate smaller blaze that erupted across town at another construction site overnight, was under investigation, she added.

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The fire disrupted morning rush-hour traffic in the city and closed an eight-mile stretch of freeway, according to the California Highway Patrol.

The structure that burned, two stories of concrete beneath five floors of wood framing, occupied an entire city block near the intersection of two freeways, Main said.

Hundreds of firefighters battled the blaze for several hours in downtown LA. Photo: Damian Dovarganes/AP

Flames spread to an adjacent 16-foor high-rise office tower, consuming about three stories of that building. Fourteen floors sustained smoke, fire or water damage, and the building was closed for the day, Ms Main said. A second, smaller neighboring office building sustained heat damage, but firefighters kept flames from encroaching on that structure, she said.

Fire department spokesman David Ortiz said flames spread across the freeway at one point before the blaze was contained.

Fire Department spokesman Jaime Moore told The LA Times the office tower sustained fire and sprinkler water damage.

The fire in downtown Los Angeles. Photo: Mariana R on Twitter

"Large windows gave under the amount of heat," Moore said, adding that sprinklers activated on six of its floors. "There was active fire on three floors."

Mr Moore said firefighters managed to get inside both buildings and prevent the fires from spreading.

The fire in downtown LA. Photo: Mariana R on Twitter

Mr Moore said the apartment complex covered an entire block. The bottom two storeys were made of concrete; the upper five floors were made of wood frame. Much of the structure was lost.

"This is a significant building that has sustained serious damage," he said.

A series of dense, upscale apartment complexes have been built over the last decade around the 101-110 freeway interchange, and another one, known as the DaVinci, was currently under construction.

"Experience urban-style elegance and European living in the heart of downtown Los Angeles. The Da Vinci, the newest member of the Renaissance collection, will offer fifteen unique floor plans to match your lifestyle," the project's website said. "Indulge in world-class amenities, such as a state of the art fitness facility, a full size indoor basketball court, a residential lounge, a library, a theatre and a business centre."

The building was in the news earlier this year when the developer sought a pedestrian bridge that would link the DaVinci to other complexes in the area and offer residents a direct route into downtown attractions.

The developer told the city that transients living under the nearby 110 Freeway pose a safety threat to his future renters. The bridge proposal faced criticism from some in downtown. But in May, the City Council approved it.

Developer Geoffrey Palmer's company is known for the Orsini, the Medici and other faux-Italian apartment buildings that have risen along the four-level interchange in downtown L.A. The complexes have been part of the revitalization of downtown, thought critics have complained about the design and size of the buildings.

When the fire broke out , flames could be seen for kilometres, with some people on social media capturing shots from as far away as the Hollywood Hills. Other photos and video captured flames racing into the air as motorists drove by.

Mr Moore said it might take some time to reopen the northbound 110 Freeway. Besides having emergency vehicles parked on the roadway, he said, there is some scaffolding from the building still standing and officials are worried it could collapse onto the freeway.

Video from KTLA showed that heat from the fire burned freeway directional signs and melted the plastic traffic bumps embedded into the roadway. The video showed much of the apartment building burned to the ground, but its fire escape was still standing.

LA Times, Reuters