It could take another 10 days to put out a huge fire at a tyre dump near Madrid, regional authorities said Monday, though residents have already been allowed to return to the area.

Some 10,000 people living in the town of Sesena near the Spanish capital evacuated their homes Friday after the dump went up in flames after a suspected arson attack.

Spanish authorities allowed the residents to return home Saturday, saying that the toxic fumes billowing from the rubber heap posed less of a risk.

The regional Castilla-La Mancha authorities in a statement estimated that "when the fire is completely extinguished (between a week and ten days) there will still be 30,000 tonnes of tyres able to be recycled and reused".

The blaze broke out before dawn last Friday in the dump that stretches over 10 hectares (25 acres) -- the equivalent of about 10 rugby fields -- and straddles the Castilla-La Mancha and Madrid regions.

A massive black cloud of fumes billowed into the air, prompting widespread health concerns. The Castilla-La Mancha government had warned that the smoke was "toxic."

The massive stack 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, and it grew from then on.

The dump was declared illegal and environmental groups had long warned that it posed a health hazard, but authorities only recently started acting on the problem, inviting bids to empty the site and destroy the tyres at the end of last year.

Francisco Martinez Arroyo, Castilla-La Mancha's top environment official issued reassurances at a press conference with his Madrid counterpart on Monday.

"The fire has been mastered and we are concentrating on the air quality," Arroyo said, stressing that the blaze was probably the result of arson.

The European Commission has requested information from the Spanish authorities on landfills and dumping sites.

In a bid to alleviate health concerns, Madrid's emergency services have tweeted that air quality measuring stations in the area continued to give normal readings.

But environmental group Ecologists in Action has warned that the stations were not set up to measure all the pollutants being released from the mass burning of tyres -- an atypical event.

These include polycyclic aromatic hydrocarbons (PAHs), compounds considered dangerous and in some cases cancer-causing.