"I myself have also experienced a part of Germany being occupied by the Soviet Union," Merkel, who grew up in communist East Germany, said as she arrived at a tense NATO summit.

"I am very glad that we are united today in freedom as the Federal Republic of Germany and that we can therefore also make our own independent policies and make our own independent decisions."

Trump said on Wednesday that Germany was a "captive" and "controlled" by Russia because of the country's dependence on Russian gas which is set to increase with the construction of a new pipeline.

Merkel also addressed repeated criticism from Trump over Germany's military spending, which is below the target level agreed by NATO members.

"Germany owes a lot to NATO," Merkel added. "The fact that reunification has taken place also has a great deal to do with NATO, but Germany is also doing a great deal for NATO.

"We are the second largest provider of troops, we put most of our military capabilities at the service of NATO," she said.

NATO members agreed in 2014 to aim to raise their annual military spending to the equivalent of 2.0 percent of gross domestic product.

Figures released from NATO on Tuesday showed Europe's biggest economy spent just 1.24 percent of GDP on defence, compared with 3.5 percent for the US.

Under new plans, Germany has announced its intention to raise its defence spending by 80 percent over the next decade.

But Trump dismissed this as not enough on Wednesday morning.

"These countries have to step it up, not over a 10-year period, they have to step it up immediately," he said as he met with NATO chief Jens Stoltenberg.

"Germany is a rich country. They talk about that they can increase it a tiny bit by 2030. Well, they could increase it immediately tomorrow and have no problem. I don't think it's fair to the United States," he added.