Tunisian Prime Minister Hichem Mechichi on Saturday instructed his interior and justice ministers to cooperate with French authorities following the deadly knife attack in Nice allegedly perpetrated by a Tunisian.

Brahim Issaoui, 21, is suspected of brutally killing three people in Thursday’s attack at the Notre-Dame Basilica in the southern French city, and French authorities are investigating whether outside help was provided.

Mechichi met Interior Minister Taoufik Charfeddine and Justice Minister Mohamed Boussetta on Saturday and reiterated Tunisia’s "absolute” condemnation of the "brutal and cowardly” attack.

"The interior and justice ministers must focus their full attention on the investigation into the circumstances of this terrorist act and fully cooperate with French investigators,” he said in a statement.

French police are currently holding three people for questioning in the investigation, which is focusing on two telephones found on the suspect after the attack.

Tunisia, which on Thursday said it launched its own investigation into the Nice killings, said that Issaoui was not on any "terrorist” list in the north African country.

But he had a "criminal record for common-law offences [such as] violence and drugs”, Mohsen Dali, the deputy attorney general at the Court of First Instance in Tunis, told AFP.

Issaoui, he said, left Tunisia clandestinely on September 14, making his way to the Italian island of Lampedusa — a major stepping stone for illegal migrants seeking to make a new life in Europe.

His family, in an interview with AFP, said he had called on the evening of October 28, the day before the attack, telling them he had just arrived in France and that he intended to find work.

Tunisian authorities arrested two people on Friday after a video posted on social networks carried a claim of responsibility for the Nice attack by an unknown group, according to Dali.

Terror experts said the claim was not credible.

Issaoui was shot by French police multiple times and is currently in a grave condition in hospital. Investigators have been unable to question him and his precise motivations remain unclear.