The US will be “unwavering” in its support for Nato, vice-president Mike Pence told European leaders at the Munich Security Conference.
In the first major foreign policy address for the Trump administration, Mr Pence said the US would “stand with Europe today and every day”.
But he told the gathered leaders that European countries were “failing to pay their fair share” on defence.
That failure “erodes the foundation of our alliance”, he said.
Apart from the US, only four other nations had met a commitment to spend 2% of GDP on defence,
“The time has come to do more,” he said.
President Donald Trump warned before taking office that the US might not uphold its commitment to come to the defence of Nato allies who were not perceived to have contributed enough financially.
Mr Pence went on to say that the US would “continue to hold Russia accountable, even as we search for new common ground, which as you know, President Trump believes can be found”.
Mr Pence said Russia must honour the Minsk peace accords on Ukraine and de-escalate its military operations in the east of the country.
His comments came amid huge controversy in the US over ties between the Trump administration and Russia.