A passenger bus caught fire and exploded in the heart of Rome on Tuesday, witnesses said, sending a plume of black smoke over the city center in the ninth such blaze this year.

Local transport authority Atac said the driver and passengers all managed to evacuate the bus, which came to a halt near the popular Trevi Fountain, before the flames took hold. No-one was injured and there was no suggestion of foul play.

In 2017, some 20 Rome buses were destroyed or damaged by fires.

Italian media have blamed the succession of blazes on Rome's aging bus fleet and on poor maintenance. Atac, which is struggling under a mountain of debts, says it has upped its repair work and is tackling the problem.

A former head of Atac said last year the company was suffocating under some 1.3 billion euros ($1.54 billion) of debts and should declare bankruptcy.

According to an internal Atac report, 36 percent of all the company's buses are in garages because they have broken down or are undergoing maintenance, with the figure rising to 50 percent for the city's creaking fleet of trams.