A preliminary report on the helicopter crash that killed Iranian President Ebrahim Raisi on Sunday has found no evidence of criminal activity, state media say.

The aircraft caught fire after hitting an elevated area and no traces of bullet holes have been detected among the wreckage, the army report says.

The helicopter had been flying on a pre-planned route and did not leave the designated flight path, it states.

The military committee investigating the crash added that more details would come as the inquiry advanced.

President Ebrahim Raisi was buried in his home city of Mashhad on Thursday, four days after the helicopter went down while returning from a dam inauguration on the border with Azerbaijan.

The aircraft - a decades-old, US-made Bell 212 - hit a mountainside as it flew toward the north-western city of Tabriz in heavy rain.

"Nothing suspicious has been observed in the control tower's conversations with the flight crew," the preliminary report said.

It added: "Signs of gunshot or similar were not observed in the wreckage."

The aircraft was found by drones on Monday but the complexity of the area, fog and low temperatures hindered the work of rescue teams, according to the report.

A statement from the military broadcast on Thursday said the final communication from the presidents helicopter was recorded about 90 seconds before the crash.

The 63-year old president was travelling with Foreign Minister Amir-Abdollahian, three other officials and the helicopters crew of three. There were no survivors.

Tehran has announced five days of mourning. Tens of thousands of people have taken part in the funeral processions.

Raisi, a hardline cleric, was seen as a potential successor to Iran's supreme leader Ayatollah Ali Khamenei.

Acting President Mohammad Mokhber will serve until an election is held onJune 28. (BBC)