At least 90 people were killed in a fire at a petrol station in Ghana's capital, Accra, as they sought shelter from heavy rains that caused widespread flooding, emergency services said Thursday.

The fire broke out at the filling station in the Kwame Nkrumah Circle area of the city late on Wednesday night and is thought to have spread from a nearby residence.

Ghana's President John Dramani Mahama described the loss of life as "catastrophic and almost unprecedented", as he toured the scene of the disaster on Thursday morning.

"A lot of people have lost their lives and I am lost for words," he told reporters.

"As at now, the confirmed figure is hovering around 90 but I know it will be more than that," national police spokesman Arthur Cephas told AFP.

"The relevant agencies are doing the tally."

Rescue officials were working to clear the area and bodies were piled onto flat-bed trucks, covered with tarpaulin sheets, as police in high-visibility vests stood guard at the scene.

Ghana National Fire Service spokesman Billy Anaglate said victims were being discovered during the salvage operations, making an exact death toll difficult to establish.

"If we begin taking any of those numbers we have gotten and we are working and we are getting a new one, we'll begin changing the figures that we are giving," he added.

Local hospitals said morgues were full and security officials said the death toll was likely to rise, as fire investigators picked through the charred debris to determine what caused the inferno.

Dozens of motorcycles were seen burnt and the fire is also thought to have engulfed a bus full of passengers that was waiting on the forecourt, an AFP reporter at the scene said.

Local residents said many people had sought refuge under the filling station canopy from days of heavy rains that have engulfed Accra.

Flood waters had reached knee level on the road beside the Ghana Oil Company (GOIL) petrol station before the fire, which caused an explosion at the pumps, according to eye-witnesses.

The explosion was heard and seen across the capital. It was not immediately clear exactly how the victims died, with reports that some had drowned after being knocked unconscious and falling into the flood waters.

- Blocked drains -

Mahama extended his condolences to the families of those who lost loved ones and said "precautionary measures" needed to be taken against flooding that hits the city every year.

The GOIL filling station is next to a large open drain that carries water from surrounding areas to the sea.

But like many gutters in the city, it was blocked with rubbish, causing water to spill onto the streets. Flooding is exacerbated by construction work in the city.

One witness, Edgar Wiredu, told GTV 24 television: "Because of the construction work, the whole of (Kwame Nkrumah) Circle was flooded.

"When the fire service got to the scene, they got stuck. They struggled to gain access to the scene."

At least two days of rains have caused chaos in Accra, leaving many suburbs submerged and people stranded, as roads were blocked and cars were carried away in or upturned by the flood waters.

Already sketchy power supplies had been cut to some communities as electricity sub-stations were under water, said communications minister Edward Omane Boamah.

Boamah, who called the situation a "national emergency", said the armed forces, police, fire and the National Disaster Management Organisation have been deployed to help those affected.

"The general public is kindly advised to avoid fast-moving rainwater and areas they know have big drains. Stay on higher ground, where necessary, to prevent loss of life," he added.

More rains are forecast on Thursday and Friday, with storms coming in from the east.

Accra receives its heaviest rainfall in May and June, according to the Ghana Meteorological Agency, with a mean total rainfall of 131.2 millimetres (5.2 inches) and 221.0 mm respectively.

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