US defence secretary James Mattis told members to "show support for our common defence"
The US has warned NATO it will "moderate" its commitment to the alliance unless member states boost their military spending.
During talks at NATO headquarters in Brussels, US defence secretary James Mattis stressed the Trump administration expects members to meet the target of spending 2% of GDP on defence.
Mr Mattis told members to "show support for our common defence" if they "do not want to see America moderate its commitment to this alliance".
He added: "I owe it to you to give you clarity on the political reality in the United States, and to state the fair demand from my country's people in concrete terms."
Mr Mattis' comments come after Donald Trump claimed NATO was "obsolete" and complained several nations do not contribute their share financially.
Despite the claims, Mr Mattis said the president "has strong support for NATO" and described the alliance as a "fundamental bedrock" for the US.
Mr Mattis added Russia's annexation of Ukraine's Crimea region in 2014 was "sobering" and said NATO must "continue to adapt to what's being revealed to us in terms of our security challenges".
During the two-day meeting in Brussels, NATO will review its operation in eastern Europe - where it is deploying four battlegroups to the Baltic states and Poland amid fears over Russian aggression.
Leaders are also set to discuss terrorism and cyber attacks.
Earlier, NATO officials confirmed the UK remained one of five states to meet the 2% spending target - despite a think tank's claims Britain spent 1.98% of its GDP on defence in 2016.
According to a NATO assessment last July, only the UK, US, Greece, Estonia and Poland met the 2% target in 2016.
Britain was a driving force behind the obligation, which was agreed at a summit in 2014.