Donald Trump and Angela Merkel got off to a prickly start at a NATO summit as the U.S. president blasted Germany over its support for a gas pipeline from Russia, prompting a tart response from the chancellor.

"It's very sad when Germany makes a massive oil and gas deal with Russia where we're supposed to be guarding against Russia and Germany goes out and pays billions and billions of dollars a year to Russia," Trump said before meeting with NATO Secretary General Jens Stoltenberg on Wednesday morning.

Trump pressed on: "If you look at it, Germany is a captive of Russia, because they supply —- they got rid of their coal plants, got rid of their nuclear, they're getting so much of the oil and gas from Russia. I think it's something NATO has to look at."