European Council President Donald Tusk (L) speaks to US President Donald Trump (R) as he welcomes him at EU headquarters.

European Council President Donald Tusk is appealing to President Donald Trump to remember who his allies are as the U.S. leader prepares to meet with Russian President Vladimir Putin.

Speaking as he signed a joint EU-NATO declaration Tuesday with alliance Secretary-General Jens Stoltenberg, Tusk said in a message to Trump that "it is always worth knowing who is your strategic friend and who is your strategic problem."

With Trump routinely criticizing his NATO allies for failing to spend enough on their defense budgets, Tusk recalled that the Europeans are spending more than Russia and as much as China.

Tusk urged the Europeans to increase spending as they have promised, but said: "Dear America, appreciate your allies, after all you don't have all that many."

Trump meets Putin on July 16.