The rallying cries were the latest in a slew of increasingly alarming warnings from European officials, who have been calling for a "wake-up call" on defence since Moscow's tanks rolled into Ukraine in 2022.
Trump has ramped up the pressure by warning Washington's European allies that he could withhold US protection, calling for NATO to more than double its defence spending target.
"President Trump is right to say that we don't spend enough. It's time to invest," EU foreign policy chief Kaja Kallas said in a keynote speech at a conference in Brussels. "The United States, they are our strongest ally, and must remain so."
"The EU's message to the US is clear, we must do more for our own defence and shoulder a fair share of responsibility for Europe's security," she said.
EU countries have increased their military budgets since Russia launched its full-scale invasion of Ukraine in 2022.
But politicians acknowledge they will have to go further as they struggle to match Moscow's vast military output.
"Russia poses an existential threat to our security today, tomorrow and for as long as we under-invest in our defence," said Kallas, a former prime minister of Estonia.
"Many of our national intelligence agencies are giving us the information that Russia could test EU's readiness to defend itself in three to five years. Who else are we listening to?"
Speaking separately at the European Parliament in Strasbourg, Polish Prime Minister Donald Trump insisted that "if Europe is to survive, it needs to be armed".
The leader of Poland, which spends proportionally more on defence than any NATO ally, urged fellow EU states to take seriously Trump's call to up the spending target to five percent of GDP from two percent.
"This is a time when Europe cannot afford to save on security," said Tusk, whose country took over the EU's rotating presidency this month.
- 'Language of strength' -
Kallas meanwhile said Europe's first priority must be helping Ukraine's fatigued forces fight back against the Kremlin's invasion as Russian President Vladimir Putin's war nears the three year mark.
"There is absolutely no doubt that we can do more to help Ukraine. With our help, they can also win the war," she said.
"The only language that Putin speaks is the language of strength."
Trump has sparked fears in Europe that he could force Kyiv into painful concessions in search of a quick deal with Moscow to end the conflict.
"Aggression as a foreign policy tool can never, never pay off," Kallas said, warning the United States that its key rival China was watching for any signs of weakness.
"Ukraine's security against Russia is security for us all."
She said she wanted "to look into doing even more" with over 200 billion euros ($208 billion) of Russian state assets frozen in the bloc.
So far the EU and its international partners have used the interest on the money to give Kyiv a $50 billion loan, but they have refused to touch the main assets.
- UK a 'key partner' -
Beyond the United States, Kallas said the EU needed to strengthen security bonds with former member and "key partner" Britain.
"We need a mutually beneficial relationship on security and defence," Kallas said. "A new agreement on this is a logical next step."
EU leaders are set to meet next month in Brussels with the British prime minister and the head of NATO for a discussion on bolstering Europe's defences.
Last month, NATO chief Mark Rutte said Europe must "turbo-charge" defence spending and production if it is to deter Russia from launching a bigger war in the future.
Kallas and EU defence commissioner Andrius Kubilius are set to put forward new proposals for bolstering the bloc's industry in March.
Despite a raft of existing initiatives, officials admit the EU has so far struggled to turn the rhetoric into reality and step up to the scale of the threat.
"The storm clouds of war are gathering over Europe," Kubilius said.
"We can outspend, outproduce -- and outgun Russia."
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