PARIS — Polish Prime Minister Donald Tusk piled pressure on other European leaders to massively ramp up defense spending and not clash with the U.S. on Tuesday as he flew to Paris for an emergency summit devoted to Ukraine's future and European rearmament.

"I will ask the prime ministers gathered in Paris today directly, are they ready to make a serious decision?" Tusk said on his way to the gathering, referring to defense spending. "Poland is unfortunately an exception to the rule in Europe at the moment. This absolutely has to change."