He said this in a BBC Ukrainecast podcast.
He admitted that "the war will be lost if the allies do not intervene." But when asked whether Western countries should send troops to Ukraine, Cameron said: "No".
According to him, NATO troops should not be a target for Russian dictator Vladimir Putin.
The minister said that next week he will make his second visit to the United States since becoming Foreign Minister and will call on Congress to provide more financial support that "can change the course of events" in Ukraine.
"If we can get that money out of the US Congress, if we can get Ukraine the arms they need, if we can show Putin that he can't outwait us and that Ukraine is going to fight back and win back more of its territory. If we can do all those things, whoever gets in in November (referring to the US presidential election - ed.), we'll look at the situation in Ukraine and... look at more and more Nato members spending 2% of their GDP on defence and say, this is a success story. I want to invest in this success," Cameron said.
He noted that he is ready to cooperate with any president who comes to power in the United States.