BRUSSELS - European Council President Donald Tusk on Tuesday appealed to the Trump administration to respect and appreciate its allies.

"Dear America, appreciate your allies, after all, you don't have that many," Tusk said, after the European Union (EU) and the North Atlantic Treaty Organization (NATO) signed a joint declaration aimed at enhancing military cooperation.

"And, dear Europe, spend more on your defense, because everyone respects an ally that is well-prepared and equipped," Tusk said following the signing ceremony.

"Money is important, but genuine solidarity is even more important," stressed Tusk, adding that he hopes US President Donald Trump keep that in mind when they meet at the upcoming NATO summit.

Featuring high on the EU-NATO declaration are military mobility, counter-terrorism, strengthening resilience to chemical, biological, radiological and nuclear-related risks, and promoting the women peace and security agenda.

The declaration also dropped a hint that European members of NATO will increase their spending on defense.

"For EU Member States, we welcome political agreement to give higher priority to security and defense in the forthcoming discussions on the next long-term EU budget," read the joint declaration.

The declaration, the second of its kind after a two-year hiatus, was inked by NATO Secretary General Jens Stoltenberg, Tusk, and European Commission President Jean-Claude Juncker in Brussels on the eve of the NATO summit.

The summit, slated for July 11-12, has been in the shadow of Trump's long-simmering discontent over the military bloc's burden-sharing.

Ahead of his departure for the summit, Trump has blitzed NATO allies with a few rounds of tweets.

"Getting ready to leave for Europe. First meeting - NATO. The US is spending many times more than any other country in order to protect them. Not fair to the US taxpayer," he tweeted Tuesday.

"On top of that, we lose 151 Billion (US dollars) on Trade with the European Union. Charge us big Tariffs (& Barriers)!" he added.

The US president has long been scolding the NATO countries which have not met the 2-percent-of-GDP benchmark on military spending.

"NATO benefits Europe far more than it does the US," Trump claimed in a tweet on Monday.