Nigeria's environmental agency has launched an urgent investigation after a strange white, foamy substance appeared on a major highway, sparking a public spectacle and raising safety concerns.

The National Environmental Standards and Regulations Enforcement Agency (Nesrea) has warned people to keep their distance from the unidentified substance along the busy Kaduna-Abuja Expressway, linking the main city in the north to the federal capital.

Hundreds of motorists have been stopping to photograph it since it appeared a few days ago, with many sharing images on social media.

Some speculated it might be a chemical spill, while others appeared to revel in the novelty of the scene.

A regular driver on the route, Isah Aminu, told the BBC that he had initially been told by another driver that "clouds had fallen from the sky".

"That was the first information many of us got - that it's clouds from the sky," he said. "But like many, I also stopped to see it and it looked like snow,” he said.

Aminu said he was afraid to touch the foam but observed many others getting close to it.

"I think the authorities ought to come out to explain what it is so that the confusion among people will die out," he added.

Nesrea confirmed to the BBC that its team from the Kaduna State Field Office, led by State Co-ordinator Hena Dangari Emmanuel, had visited the site.

The agency said samples of the foamy substance, as well as soil and water from a nearby stream, had been collected for laboratory tests to determine their composition and potential environmental impact.

The agency has advised members of the public to avoid contact with the substance, the water, and the surrounding soil until the investigation is concluded, in order to prevent possible exposure to hazardous materials.

"We have to know what it is first before we can tell people how to handle it or what the dangers are," a spokesperson said.

Preliminary findings indicated that a spillage occurred after a lorry was involved in an accident. However, the exact nature of the lorry's cargo remains unclear, the spokesperson added.