PARIS — After a string of attacks across France that has heightened concerns about Islamic militancy, Prime Minister Manuel Valls said on Tuesday that hundreds of additional military personnel would be ordered onto the streets to reinforce a routine deployment of security forces.

“There is a terrorist threat in France,” Mr. Valls told a news conference in Paris. “It is undoubtedly the main challenge of our time.”

But, seeking to reassure a jittery nation unsettled by fears of militancy linked to the jihadist campaign in Syria and Iraq, Mr. Valls said that 200 to 300 more military personnel would be deployed, in addition to 780 already on the streets as part of routine year-end precautions. He indicated that the soldiers’ mission would be to guard against copycat attacks inspired by the three assaults.

“Vigilance, calm, determination. These are the key points,” Mr. Valls said, speaking after successive attacks in the central town of Joué-lès-Tours on Saturday, in Dijon on Sunday, and in Nantes on Monday.