President Donald Trump has kicked off a four-nation European tour by bashing the NATO alliance as unfair to US taxpayers.

During his presidential campaign, Trump suggested he might come to the defense only of NATO nations that fulfilled their obligation to spend at least 2% of gross domestic product on defense.

Also looming over Trump's meeting with the US's NATO allies is his scheduled summit with Russian President Vladimir Putin at the end of the week.

President Donald Trump has kicked off a four-nation European tour by bashing NATO as unfair to US taxpayers.

Combine with his pending meeting with Russian President Vladimir Putin in Finland, Trump has allies fretting over the risk of damage he could do to the decades-old NATO military alliance.

"Getting ready to leave for Europe," Trump tweeted on Tuesday. "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)!"

Trump has been pressing fellow NATO countries to fulfill their goal of spending 2% of gross domestic product on defense by 2024. During his presidential campaign, he suggested he might come to the defense only of NATO nations that fulfilled that obligation. He continues to criticize NATO countries that spend less than that share.

NATO's Article 5 says any member of the alliance can invoke a mutual defense if it's attacked. The US is the only nation to have invoked that clause, doing so after the September 11, 2001, terrorist attacks. NATO allies responded with nearly two decades of support for US operations in Afghanistan.

Still, Trump complained Monday that the US was "spending far more on NATO than any other Country."

"This is not fair, nor is it acceptable," Trump added, insisting that NATO benefited Europe "far more than it does the U.S."

"On top of this the European Union has a Trade Surplus of $151 Million with the U.S., with big Trade Barriers on U.S. goods. NO!" he protested.

NATO estimates that 15 members, or just over half, will meet the benchmark by 2024 based on current trends.

Trump expected to encounter protests in the UK

Also as part of this trip, Trump, who has compared the Brexit vote to leave the European Union to his own election, will be making his maiden presidential trip to Britain at a fraught time for British Prime Minister Theresa May. Two Brexit proponents in her Cabinet, Foreign Secretary Boris Johnson and Brexit Secretary David Davis, resigned within hours of each other this week in protest of her plan.

Trump's visit is expected to attract large protests in London and elsewhere in Britain.

Trump and Putin's meeting raises eyebrows

Trump with Russian President Vladimir Vladimir Putin. Thomson Reuters

Trump's weeklong trip to Europe will continue with a stop in Scotland before ending with a sit-down in Helsinki with Putin.

The meeting will be closely watched to see whether Trump will rebuke or embrace Putin, who has repeatedly denied meddling in the 2016 election, something the US intelligence community says Russia did with the goal of helping Trump.