PARIS (Reuters) - French troops are involved in an operation near Niger s border with Mali on Thursday, the French army said, a day after an attack that killed three U.S. Army Special Forces members.  French army spokesman Colonel Patrick Steiger told a news conference in Paris that its regional Barkhane force had been asked to support a counterattack, after Niger and U.S. troops were ambushed by suspected Islamist militants overnight.  He said fighter jets had been dispatched to the area and French helicopters had been used to evacuate the wounded. No French soldiers had been killed in the initial attack, Steiger added. 