French President Francois Hollande promised early Saturday morning that France would respond to terrorist attacks that killed more than 120 people with a "pitiless" war against the group responsible.

"We are going to lead a war which will be pitiless," he said at the Bataclan, the site of one of the attacks, according to the Guardian.

"Because when terrorists are capable of committing such atrocities, they must be certain that they are facing a determined France, a united France, a France that is together and does not let itself be moved, even if today we express infinite sorrow," Hollande added.

Hollande's remarks came after what appears to be the biggest terrorist attack ever on French soil, and some emerging signs that the Islamic State is responsible.

They also may pose a challenge to the Obama administration, which has faced increasing criticism about whether its plan to "degrade and ultimately destroy" ISIS is working fast enough. Secretary of State John Kerry was in Vienna Friday for more talks about how to handle the Syrian civil war, which has been credited with creating the conditions for the Islamic State to thrive there.

"These are heinous, evil, vile acts," Kerry said in response to the attacks. "Those of us who can must do everything in our power to fight back against what can only be considered an assault on our common humanity."