Brussels – The European Union and the United States have lashed out at one another in a row over defence spending, one day before a NATO summit gets under way in Brussels.

In the build-up to the summit, US President Donald Trump has been calling on European allies to increase their defence expenditure, arguing that they are free-riding on Washington's military might.

NATO allies have pledged to spend 2 per cent of gross domestic product on defence, a goal that only eight of them are expected to reach this year.

"Not fair to the taxpayer"

"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," Trump tweeted early Tuesday.

Shortly thereafter, European Council President Donald Tusk responded to Trump for "criticizing Europe almost daily."

"Dear President Trump: America does not have, and will not have a better ally than Europe," Tusk said at a press conference. "Today Europeans spend on defence many times more than Russia, and as much as China," he added.

"Dear America, appreciate your allies, after all you don't have that many," Tusk warned, while calling on Europe to increase its defence spending since "everyone expects an ally that is well prepared and equipped."

Only eight members meet NATO target

Increasing defence spending by European NATO allies and Canada was not about pleasing the US, but about ensuring defence and security in the face of growing threats, NATO Secretary General Jens Stoltenberg said.

"European allies and Canada should not increase defence spending to please the United States," Stoltenberg said. "They are increasing defence spending because it's in their own security interest to do so."

Stoltenberg said that the US, Estonia, Greece, Britain, Latvia, Lithuania, Poland and Romania were the eight allies expected to meet the defence spending target this year.

Tusk also addressed his message to Trump in a tweet, following which the US president tweeted minutes later: "NATO countries must pay MORE, the United States must pay LESS. Very Unfair!"

Tusk further sought to "dispel" the argument that the US alone protects Europe, noting that the continent was the first to respond on a large scale when the US was attacked on September 11, 2001, and has lost 870 soldiers in the ensuing war in Afghanistan.

"Dear Mr President, please remember about this tomorrow when we meet at the NATO summit, but above all when you meet [Russian] President [Vladimir] Putin in Helsinki. It is always worth knowing who is your strategic friend, and who is your strategic problem," Tusk said.

Pledging closer cooperation

Trump is due to arrive in Brussels at around 9:30 pm (1930 GMT) on Tuesday. The NATO summit, on Wednesday and Thursday, is his first leg of a European tour that also takes him to Britain and then Helsinki, for his highly anticipated meeting with Putin.

Tusk's comments came after he signed a joint document alongside Stoltenberg and European Commission President Jean-Claude Juncker, pledging closer NATO-EU cooperation.

Trump's implicit threat that the US could scale back its military presence in Europe has boosted the argument that the continent must do more to strengthen its own defence capabilities, at a time when Russia is acting provocatively to the east.

Twenty-two members of the NATO alliance are also part of the EU. Relations between the two organizations have proved challenging in the past, due in large part to tensions between NATO member Turkey and EU member Cyprus, but those strained ties turned a corner in 2016.

"We are proud of what has been achieved together so far. But we can do more," the joint EU-NATO statement says. In particular, the two sides pledged to cooperate on measures to ease military transports across Europe – for example by upgrading roads and bridges while reducing bureaucracy at national borders – as well as on counterterrorism and improving resilience to chemical, biological and nuclear risks.

The statement also stresses that NATO will "continue to play its unique and central role as the cornerstone of collective defence," in response to US concerns that EU defence initiatives could create parallel structures. "Europe is taking more responsibility for its security," Stoltenberg said, adding: "This is complementary, not an alternative to NATO."