Nato members have pledged to spend at least 2% of the value of their economies – measured by GDP – on defence per year by 2024.
But speaking at an event in Brussels, the former Dutch prime minister said “a lot more” would be needed as danger “[moves] towards us at full speed”.
He said European members had spent upwards of 3% of GDP on defence during the Cold War.
“If we don’t spend more together now to prevent war, we will pay a much, much, much higher price later to fight it,” he said.
He added that Russia’s economy was “on a war footing”, with its defence spend by 2025 set to be “a third of Russia’s state budget – and the highest level since the Cold War”.
While the average defence spend for Nato members in Europe and Canada is estimated at 2%, not all meet the target.
Trump said in February that he would “encourage” Russia to attack any Nato member that fails to pay its bills as part of the Western military alliance.
Nato’s 32 members in Europe and North America agree that if one member is attacked, the others should help defend them.