A former German defense chief told CNBC Friday that it may be impossible for his country to meet NATO spending targets and questioned whether President Donald Trump would be happy with any outcome on the issue.

Trump has repeatedly called on NATO allies like Germany to increase contributions toward defense, often criticizing them for not complying with a commitment to spend 2% of their gross domestic product (GDP). But Karl-Theodor zu Guttenberg, the former German defense minister, doubted whether Germany would ever reach that target.

"Will Donald Trump ever be happy? Probably not, even if they come to these numbers, which I think is more or less impossible, he will just find the next thing to throw at Germany, or not even at Germany, whomever pleases him at that very moment," he told CNBC's Geoff Cutmore in Salzburg, Austria.

The relationship between the United States and Germany has been somewhat tense since Trump took over at the White House, especially in areas such as trade and foreign relations. Zu Guttenberg said the most significant thing for the current German defense minister, Annegret Kramp-Karrenbauer, is to show that Germany is "an extremely reliable partner in military terms" and that it is willing to take responsibility in "even nastier places around this globe."

"And then of course come closer to the threshold of 2%. But it is quite hard," he said, adding that it would be difficult to get such a rise in defense spending through the German parliament.