A layer of sand has covered the Senegalese capital Dakar, an unusual phenomenon that poses a "high" risk for vulnerable people, according to the National Weather Agency.

Dakar residents, who had dropped the habit of wearing masks to protect against Covid-19, put them back on Friday, to offer some protection against the dust cloud that has enveloped the capital and hit other regions since Thursday.

The sky was visibly full of particles which settled on the bonnets of cars.

"It is a layer of sand arriving from Mauritania," a neighbouring West African country with a large desert that is regularly hit by sandy winds, Ngor Ndiaye, an official at the National Agency for Civil Aviation and Meteorology (Anacim) told AFP.

Dakar has experienced this phenomenon before, but not normally this early in the year or this severe, he added.

Anacim warned on social networks of "very bad" air quality in Dakar, with concentrations of particles exceeding 900 micrograms per cubic metre, nearly 20 times the maximum level recommended by the World Health Organisation over a 24-hour exposure.

The health risk is "high", particularly for at-risk people such as those suffering from respiratory diseases, young children and the elderly.

The dust should start to dissipate gradually from Saturday evening, the weather agency predicted.