Donald Trump on Tuesday pushed NATO members to boost their defense spending to five percent of GDP, underlining his long-standing claims that they are underpaying for US protection.

"They can all afford it, but they should be at five percent not two percent," the incoming US president told reporters.

"Europe is in for a tiny fraction of the money that we're in," Trump said. "We have a thing called the ocean in between us, right? Why are we in for billions and billions of dollars more money than Europe?"

Trump has long been skeptical of NATO, the cornerstone of security in Europe since World War II, and last month reiterated a familiar threat to leave the alliance if its members did not step up spending.

The transatlantic alliance's 32 countries in 2023 set a minimum level for defense spending of two percent of gross domestic product, and Russia's war in Ukraine has jolted NATO to strengthen its eastern flank and ramp up spending.

Trump is not the only top official to call for an increase -- NATO chief Mark Rutte likewise said last month that "we are going to need a lot more than two percent." Rutte also warned that European nations were not prepared for the threat of future war with Russia, calling on them to "turbocharge" their defense spending.

In his remarks on Tuesday, Trump claimed that President Joe Biden decided Ukraine should be able to join NATO, suggesting that this helped lead to Russia's all-out invasion in February 2022. "Somewhere along the line, Biden said, no, they should be able to join NATO. Well, Russia has somebody right on their doorstep, and I could understand their feeling about that," Trump said.

NATO allies in reality agreed to Ukrainian membership in 2008 -- when Republican president George W. Bush was in office -- while the United States and Germany have more recently backed away from allowing Kyiv to join out of fear it could drag the alliance into a war with Russia.

The conflict "should have never been started," Trump said Tuesday, adding: "I guarantee you, if I were president, (the) war would have never happened."