France is stepping up security after three days of violent lone-wolf attacks.br br The incidents have left about 30 wounded and reignited fears of attacks by Islamist radicals.br br French Prime Minister Manuel Valls says the country faces a terrorist threat.br br (SOUNDBITE) (French) FRENCH PRIME MINISTER, MANUEL VALLS, SAYING:br br "There is, as you know, a terrorist threat to France. We -- along with the Interior Minister -- have been saying it for a long time. It's without a doubt, the major challenge of our times."br br The most recent attack came Monday night in Nantes where a man plowed his van into a crowded Christmas market, injuring at least 10 people.br br On Saturday a stabbing wounded three police officers, and on Sunday another man drove into crowds.br br In these first two incidents, the perpetrators yelled "Allahu Akbar".br br Valls says investigating the attacks is a top priority.