Nato Secretary-General Mark Rutte demanded that the allies put forward “clear, concrete and credible plans” to reach the organisation’s spending targets at its annual summit in Ankara.
Mr Rutte made the remarks in the Turkish capital ahead of a two-day summit starting on Tuesday.
It comes at a crucial time for the alliance with the United States, scaling down its security role in Europe. Washington has been pressing allies to shoulder more of the spending burden.
The 32 nations agreed last year to invest 5% of their gross domestic product on defence — 3.5% on their defence budgets and 1.5% on roads, bridges and ports so troops and equipment can move faster in times of conflict.
Spain endorsed the goal but said that it could fulfil Nato’s security requirements without spending so much.
Some countries are still struggling to meet the alliance’s old target of 2% of GDP.
Asked what would happen to allies that don’t have a clear plan, Mr Rutte said: “If one or two still have to be convinced, we have ways to do that.”
He did not elaborate.
US ambassador to Nato, Matthew Whitaker, suggested last week that the United States does have something in store for those who do not step up, but he declined to say what that might be.
On spending among European allies and Canada, Mr Rutte said that “the evidence we see so far is impressive”.
He said that Nato estimates that they will invest a combined 258 billion dollars (£193 billion) more in defence in 2025 and this year than they have in previous years.
But the numbers might not be enough to satisfy the Trump administration.
President Donald Trump has repeatedly lashed out at US allies over defence spending, and in the past, threatened not to come to the defence of any member not doing enough.
Mr Trump has called for “loyalty” from Nato allies, after some of them declined to allow the use of their bases in the US-Israeli war on Iran.