NANTES - A man driving a van ploughed into a crowded Christmas market in Nantes in western France on Monday evening, injuring at least 10 people, authorities said.Five people including the driver suffered serious injuries and one of the victims was in a critical state, they said.After mowing down shoppers and passers-by in the popular square in the center of the city, the driver stabbed himself several times with a knife, according to the interior ministry.A ministry spokesman said it was too early to determine the man's motives. Nantes prosecutor Brigitte Lamy told reporters on the scene that he had made no religious claim and appeared to have acted alone.Local newspaper Ouest France had earlier reported, citing a police officer and witnesses, that the driver shouted "Allahu Akbar" ("God is greatest" in Arabic). But several witnesses denied to Reuters that he pronounced those words.The incident comes a day after a man rammed his car into crowds in the eastern city of Dijon, injuring 13 people and shouting "Allahu Akbar". And on Saturday, a man was shot after he stabbed and wounded three policemen in central France while yelling those same words."We are all concerned about this series of tragedies," Prime Minister Manuel Valls said in a statement, calling on people to remain calm and vigilant.France is on high alert for attacks on its soil following calls from Islamic State (IS) militants to target the citizens of countries attacking IS positions in Iraq.However, prosecutors said the Dijon driver suffered from mental illness, had received psychiatric care over 150 times in the past 13 years and had no terrorist motives.They said they had not found any link between the events in Dijon and the Saturday attack in Joue-les-Tours, which is being probed by the counter-terrorism unit of the Paris prosecutor's office.The attacker in Joue-Les-Tours, a 20-year-old Burundi national and convert to Islam named Bertrand, had written a will in which he asked God to "give him strength," said Paris prosecutor Francois Molins.Intelligence services had been closely watching Bertrand's 19-year-old brother Brice after their mother alerted authorities to the fact that Brice had embraced radical Islam, Molins said.Two days before attacking the police station, Bertrand imitated his brother in posting on his Facebook page a flag of the extremist group that calls itself Islamic State, he added.