Trump and Hillary Clinton both spoke with Fox News' Bill O'Reilly on Thursday.

Republican presidential candidate Donald Trump said in the aftermath of a deadly truck attack in France that if he's elected president he would ask Congress for a declaration of war on the Islamic State.

In an interview Thursday night on Fox News Channel's The O'Reilly Factor, Trump said, "This is war." He spoke after a truck carrying weapons and hand grenades drove onto a sidewalk in Nice, France, and plowed through people celebrating Bastille Day, killing at least 84 people.

Trump said to fight the Islamic State, which he calls a "cancer," NATO should be used "for a purpose."

It was not immediately clear Thursday night who was behind the attack.

In a separate interview on the network, Democratic candidate Hillary Clinton said the U.S. needs to "stand strongly" with France and strengthen our alliances, including with NATO, to ferret out terrorism and prevent future attacks. Clinton said she would intensify efforts to put together a more effective coalition against terrorism.

Earlier Thursday, Trump tweeted in response to the attacks: "Another horrific attack, this time in Nice, France. Many dead and injured. When will we learn?"

Another horrific attack, this time in Nice, France. Many dead and injured. When will we learn? It is only getting worse.

— Donald J. Trump (@realDonaldTrump) July 14, 2016

.@realDonaldTrump on fighting terror threat: "We will be very, very strong if I win & we will be very, very smart." pic.twitter.com/vlbhaaRI6f

— Fox News (@FoxNews) July 15, 2016

.@HillaryClinton: "As we take territory back from ISIS forces they will try to engage in other terrorist attacks..." pic.twitter.com/tg1TxFzoHK

— Fox News (@FoxNews) July 15, 2016