Donald Trump has said that if he is elected president he may abandon a guarantee of protection to fellow NATO countries.
Speaking to the New York Times, Mr Trump said the US would only come to the aid of allies if they have "fulfilled their obligations to us".
Members of NATO have all signed a treaty that says they will come to the aid of any member that is attacked.
Asked about Russian aggression towards NATO countries in the Baltic region, Mr Trump suggested the US might abandon the longstanding protections offered by the US to such nations.
The divisive Republican candidate also said that, if elected, he would not pressure US allies over crackdowns on political opposition and civil liberties, arguing that the US had to "fix our own mess" before "lecturing" other nations.
He said: "Look at what is happening in our country. How are we going to lecture when people are shooting policemen in cold blood?"