A huge waste ground near Madrid where millions of tyres have been dumped was on fire Friday, releasing a thick black cloud of toxic fumes that officials worry could harm residents nearby.

The government of the Castilla-La Mancha region where the dump is located, just dozens of kilometres south of the Spanish capital, said it had activated an emergency action plan.

Firefighters and helicopters were working to extinguish the blaze, which produced a ‘toxic cloud... that could affect part of the (nearby) town of Sesena,’ which counts some 20,000 residents, the regional government added in a statement.

‘Around a fifth (of the dump) is affected, thousands of tonnes (of tyres) are piled up there,’ Luis Villarroel, an official at Madrid's firefighting department, said in a video posted on Twitter.

He urged residents nearby to close their doors and windows, and to try and stay away from the cloud of smoke emanating from the fire -- the cause of which is still unknown.

A spokesman for the national emergency services told AFP the alert was given in the early hours of Friday morning, adding that no one had been injured.

The massive pile of tyres started to form in the 1990s when a company began using the site as a temporary depot for old tyres due to be recycled.

But over the years the tyres started to pile up in this wild dump that stretches over some 10 hectares (25 acres), the equivalent of 10 rugby fields.

Environmentalists have for years warned that the dump poses a health hazard, and the town of Sesena has lived in fear of the rubber heap catching fire.

These types of blazes are notoriously difficult to put out and have been known to go on for months and even years, as tyres often continue to burn inside even if they are extinguished from the outside, and easily reignite.