His warning came as the U.S. and five Arab countries launched airstrikes on ISIS targets in Syria. Cameron said he hasn't "ruled out" British airstrikes.

"It has oil, it has money, it has territory, it has weapons. And there's no doubt in my mind it has already undertaken and is planning further plots in Europe and elsewhere, specifically in Belgium, in Brussels," Cameron said. "There are other plots they have been attempting including in my own country in order to kill and maim innocent people. And the same applies to the United States of America.'

British Prime Minister David Cameron told NBC Nightly News anchor Brian Williams Tuesday that he is certain ISIS is plotting attacks in Europe. "These people want to kill us," Cameron said. "They've got us in their sights and we have to put together this coalition ... to make sure that we ultimately destroy this evil organization."

"We very much support the actions the Americans and others have taken. But we need to recognize this is going to take time, it's gonna take real resolution and resolve," he said. "And we need to make sure we are working very closely with those on the ground, the Iraqi forces, the Kurdish Peshmerga forces, and actually the Syrian national opposition — who in the end are the ones who will help to destroy and get rid of this appalling organization."

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Cameron also spoke about his planned meeting with Iranian President Hassan Rouhani at the United Nations General Assembly, the first by a British prime minister since 1979. "I will do it not having softened in any way my views about the things that Iran has done and continues to do. I will be very clear. We think they are wrong to have this nuclear weapon program. We think they are wrong to support terrorist organizations."

Iran says it is enriching uranium for energy purposes and is not pursuing nuclear weapons.

Read MoreBritain raises its terrorism threat level over Syria, Iraq



"It'll be a tough conversation," he said. "I'm not saying that my enemy's enemy is my friend. I don't believe that. But the fact is if we want to have a successful, democratic, pluralistic Iraq and if we want to have a successful, democratic, pluralistic Syria — Iran can play a constructive role in helping to bring that about."

Closer to home, Cameron said that he had been "extremely nervous and worried" about the outcome of last week's referendum on Scottish independence, which ended with a no vote. "And thank heavens in the end the result was very decisive — keep the United Kingdom together," he said. "And this question is now settled for a generation or possibly, as the leader of the separatists put it, for a lifetime."