"With regards to the particular situation that the city of Nice has just experienced, I have decided to increase the threat level… to "attack alert" in order to ensure all the services are ready," Valls said, addressing the National Assembly, France's lower house of parliament.
On Tuesday afternoon, a knife-wielding attacker approached three soldiers on anti-terrorist patrol near a Jewish community center in Nice, stabbing one in the cheek and another in the arm.
Counter-terrorism measures have been stepped up in France after a series of terrorist attacks hit the Paris region in early January. Suspected Islamic extremists born in France killed a total of 17 people in three incidents that took place three days in a row starting January 7.
Earlier on Tuesday, French Interior Minister Bernard Cazeneuve announced the arrest of eight suspects near Paris and Lyon for their alleged role in recruiting citizens to join jihadist fighters in Syria.
According to Valls' remarks last month, some 1,400 people living in France are "involved in the departures for jihad" to join fighting in the Middle East.