French President Francois Hollande on Monday sought to defuse a row over allegations his government tried to alter a security report after the Bastille Day massacre in Nice.

A probe into the claim should be allowed to run its course, Hollande said, declaring the truth would be established "(by) the law and no one else."

"Truth and transparency are essential in a democracy," he added.

At the centre of the storm is Interior Minister Bernard Cazeneuve, who right-wingers accuse of security failings after France suffered its third major terror attack in 18 months.

Eighty-four people were killed when a truck driven by a 31-year-old Tunisian ploughed through a holiday crowd on the Nice seafront on July 14.

Pressure on Cazeneuve intensified when a local police officer, Sandra Bertin, accused his ministry of trying to bully her into altering a report on police deployment on the night of the attack.

French Interior minister Bernard Cazeneuve has hit back at the "grave accusations" that he pressured a senior policewoman to alter a report into security at the Nice fireworks display where 84 were killed Geoffroy Van der Hasselt, AFP/File

Prime Minister Manuel Valls waded into the affair on Monday, telling French TV the row was "purely political and aimed at destabilising the government."

While "obviously we need the truth", the row should stop, he said, defending Cazeneuve as "a man of integrity, a statesman, and a great interior minister."

Cazeneuve filed a defamation lawsuit against Bertin, saying her accusations were "likely to give birth in the public mind to the idea that the ministry and the minister have communicated false information" regarding the Nice massacre.

According to the complaint, her words "attack the honour and reputation" of the national police and the minister.

A national police report into the bloodbath had said Mohamed Lahouaiej Bouhlel forced his lorry onto the sidewalk to avoid a police barrier.

French policewoman Sandra Bertin (C) attends a wreath-laying ceremony in Nice, southeastern France on July 25, 2016 Bertrand Langlois, AFP

But Bertin, who was in charge of the video surveillance system in Nice on the night of the massacre, said on Sunday she was told to describe a police presence that did not match what she had seen on the security camera system.

Bertin said she had been "literally harassed the entire time" that she wrote the report.

"I was asked to change things," she told a news conference, adding that she was ordered "to put in the specific positions of the national police, which I had not seen on the screen".

Bertin is a strong supporter of opposition Republicans regional leader Christian Estrosi and has fiercely criticised the Socialist government through social media networks, Le Parisien newspaper reported Monday.

- 'Destruction of evidence' -

On Thursday, the Liberation daily reported that only one local police car was barring entry to Nice's famous Promenade des Anglais when Bouhlel forced his lorry onto the pedestrianised seafront and began his lethal drive.

Police monitor security cameras in Nice, southeastern France, where a row has erupted after prosecutors demanded videos of the night of the Nice attack be erased Valery Hache, AFP/File

The furore has been further fuelled by a request from prosecutors that security camera videos of the night of the attack be erased.

Local authorities claimed it amounted to "destruction of evidence" but prosecutors, who already had a copy, said it was to stop "shocking" images of the events -- in which 350 people were also hurt -- leaking out.

Former Nice mayor Estrosi has also accused the government of lying over the number of police and soldiers protecting the crowds that night.

Cazeneuve's office strongly denied Sunday that he had sent a representative to the local Nice police, "contrary to what Madame Bertin claims".

Francois Molins, the Paris anti-terrorist prosecutor who is leading the investigation, told AFP he had sent two officers to talk to police in the Nice video unit on July 15 as part of his inquiry.