Milan's San Raffaele Hospital announced Zanardi had undergone surgery "aimed at craniofacial reconstruction".

A second operation is planned to take place in the coming weeks.

A statement from the hospital said Zanardi has responded to rehabilitation sessions.

"Alex Zanardi continues in sub-intensive therapy maintained by a multidisciplinary team coordinated by the professors Luigi Beretta, director of Neurorimation, Mario Bussi, Director of Otorhinolaryngology, Sandro Iannaccone, Director of Neurorehabilitation, and Pietro Mortini, Director of Neurosurgery," a statement read.

"For several days, Alex Zanardi has been undergoing cognitive and motor rehabilitation sessions, with the administration of visual and acoustic stimuli to which the patient responds with transient and initial signs of interaction with the environment."

The hospital said it was premature to make a long-term prognosis.

Zanardi, who won two gold medals in Para-cycling at both London 2012 and Rio 2016, was placed in a medically induced coma following an accident while competing in a national handcycling race in Siena in Italy on June 19.

He was moved to a rehabilitation centre, but returned to hospital on July 25 and required a fourth brain operation.

Zanardi competed in Formula One in the 1990s.

In 2001, while competing in the Championship Auto Racing Teams series, he suffered a life-changing accident in which he lost both legs.

Following the accident, Zanardi was fitted with two prosthetic limbs and after competing at the World Touring Car Championship turned to Para-cycling.

Zanardi has won six Paralympic medals in total - four gold and two silver.

He won road time trial golds at London 2012 and Rio 2016, as well as mixed team relay gold in Brazil and a road race gold in 2012.