Donald Trump has launched a blistering attack on Germany, denouncing it as a "captive" of Russia, as he ramped up demands for NATO allies to pay more for their collective defence.
Arriving in Brussels for a two-day alliance summit, the US president said it was "totally inappropriate" that Germany was paying billions of dollars to Russia for oil and gas while spending little more than 1% of its GDP on defence.
He said the deal to build a new pipeline meant Germany was now "totally controlled" by Moscow.
German Chancellor Angela Merkel hit back, saying that having experienced life in Soviet-controlled former East Germany, she was glad they could now "determine our own policies and make our own decisions".
The barbed exchanges set the stage for another potentially stormy international summit after last month's G7 meeting in Quebec ended in angry recriminations, leading some to question US support for the postwar international order.
The president launched his tirade against Germany during a meeting on Wednesday ahead of the main summit with NATO secretary general Jens Stoltenberg.
"I think it is very sad when Germany makes a massive oil and gas deal with Russia. We are supposed to be guarding against Russia and Germany goes out and pays billions and billions of dollars a year to Russia," he said.
"We are protecting Germany, we are protecting France, we are protecting all of these countries, and then numerous of the countries go out and make a pipeline deal with Russia where they are paying billions of dollars into the coffers of Russia. I think that is very inappropriate.
"It should never have been allowed to happen. Germany is totally controlled by Russia because they will be getting 60% to 70% of their energy from Russia and a new pipeline.
"You tell me if that's appropriate because I think it's not," he said.
"I think that two world wars and the Cold War taught us that we are stronger together than apart," he said.
The president however pressed on, demanding the Germans increase their military spending "immediately" rather than over a period of years.
"Germany is a rich country. They talk about they are going to increase it a tiny bit by 2030. They could increase it immediately tomorrow and have no problem. We are going to have to do something. We can't put up with it," he said.
"Germany is a captive of Russia. They got rid of their coal plants, they got rid of their nuclear - they are getting so much of the oil and gas from Russia."
Mr Trump's comments appeared to refer to the Nord Stream 2 undersea pipeline which will bring gas from Russia to Germany's Baltic coast, bypassing Eastern European nations such as Poland and Ukraine and doubling the amount of gas Russia can send directly to Germany.
Arriving for the summit, Mrs Merkel was adamant that Germany remained independent and able to make its own decisions.
"I've experienced myself a part of Germany controlled by the Soviet Union and I'm very happy today that we are united in freedom as the Federal Republic of Germany and can thus say that we can determine our own policies and make our own decisions and that's very good," she said.
Mrs May, meanwhile, emphasised that the UK continued to meet the alliance's target of spending 2% of GDP on defence - one of just five member states to do so.
"NATO is as vital to us today as it ever has been. The UK's commitment to it remains as steadfast as ever. We show that of course.
"We lead by example," she said.
The Prime Minister's efforts to isolate Russia diplomatically following the Salisbury nerve agent attack have, however, been dented by Mr Trump's decision to meet President Vladimir Putin in the Finnish capital Helsinki following his visit to Britain.
Donald Trump set off for Europe in combative mood ahead of the NATO summit.
In a tweet posted before his departure, Mr Trump put European allies on notice that he will be taking a tough stance on their contributions to the military alliance, as well as barriers to US trade.
Getting ready to leave for Europe. First meeting - NATO. The U.S. is spending many times more than any other country in order to protect them. Not fair to the U.S. taxpayer. On top of that we lose $151 Billion on Trade with the European Union. Charge us big Tariffs (& Barriers)!
But European Council President Donald Tusk hit back with his own message on the social media site, insisting that the US has no better ally than the EU.
In a pointed apparent reference to Mr Trump's upcoming meeting with Russian President Vladimir Putin, Mr Tusk noted that European military spending "is an investment in our security, which cannot be said with confidence about Russian and Chinese spending".
Mr Trump has repeatedly complained that the US is left to provide the lion's share of funding for trans-Atlantic defence, as European states fail to meet NATO's target of spending 2% of GDP on the military.
Mr Trump said: "The US is spending many times more than any other country in order to protect them. Not fair to the U.S. taxpayer.
"On top of that we lose $151 Billion on Trade with the European Union. Charge us big Tariffs (& Barriers)!"
And he added: "NATO countries must pay MORE, the United States must pay LESS. Very Unfair!"
Minutes later, Mr Tusk tweeted: "Dear Donald Trump. US doesn't have and won't have a better ally than EU.
"We spend on defense much more than Russia and as much as China.
"I hope you have no doubt this is an investment in our security, which cannot be said with confidence about Russian & Chinese spending."
Dear @realDonaldTrump. US doesn’t have and won’t have a better ally than EU. We spend on defense much more than Russia and as much as China. I hope you have no doubt this is an investment in our security, which cannot be said with confidence about Russian & Chinese spending :-)