BRUSSELS -- President Trump confirmed Europe's worst pre-summit fears by using the NATO summit to criticize U.S. allies, blasting Germany over a natural gas pipeline deal with Russia.

"It's very sad when Germany makes a massive gas oil and gas deal with Russia where you're supposed to be guarding against Russia, and Germany goes out and pays billions and billions of dollars a year to Russia," he said.

German officials deny that the controversial Nord Stream pipeline will allow Russia to exert undue influence over their country. NATO Secretary General Jens Stoltenberg tried to keep the peace.

"We are stronger together than apart," Stoltenberg said.

"How can you be together when a country is getting its energy from the person you want protection against?" Trump responded.

"Because we understand that when we stand together, also in dealing with Russia, we are stronger," Stoltenberg said.

German Chancellor Angela Merkel and U.S. President Donald Trump are seen as they pose for a family photo at the start of the NATO summit in Brussels, Belgium July 11, 2018. Reinhard Krause / REUTERS

Later, Mr. Trump and German Chancellor Angela Merkel exchanged pleasantries for the cameras during a one-on-one meeting. But Merkel, who grew up in communist East Germany, told reporters she had "experienced myself how a part of Germany was controlled by the Soviet Union" and said her country today made "independent policies" and "independent decisions."

The NATO summit is usually a chance for member nations to reaffirm their commitment to an alliance that focuses on Afghanistan, Syria, Ukraine and other world hotspots. But Mr. Trump appeared determined to upset that tradition, demanding NATO countries spend 4 percent of their gross domestic product on defense, up from the 2 percent the U.S. has previously requested.

"We're protecting everybody and yet we're paying a lot of money to protect," the president said.

Stoltenberg said NATO should focus for now on reaching the 2 percent goal by 2024, something just five of the 29 nations have done. The U.S. is at 3.5 percent, or $685 billion last year.