NATO foreign ministers are meeting in Washington to celebrate the alliance's 70th anniversary amid rifts between members over security and trade issues.
Secretary of State Mike Pompeo opened the meeting Thursday with an appeal for unity to confront "great power" challenges from Russia, China and Iran.
He said the alliance was well positioned to move forward.
But a deepening dispute with Turkey over its planned purchase of a Russian air defence system and US demands for allies, particularly Germany, to boost their defence spending threatened to overshadow the proceedings.
Pompeo said every NATO member had an obligation to explain to its citizens the need to increase their defence budgets.
He rejected what he called "tired" arguments about public opposition to such spending.
More From This Section
"Each nation has the duty to make the case to our people ... about why these resources are important to keep not only our own countries, but our alliance, strong," Pompeo said.
In an address to Congress on Wednesday, NATO Secretary General Jens Stoltenberg acknowledged serious divisions within the alliance and called for bigger defence budgets to cope with global challenges such as Russian assertiveness, the core reason NATO was created in Washington 70 years ago this week.
President Donald Trump has questioned the value of the alliance and suggested that some members are freeloaders.
The Trump administration is threatening to stop delivery to Turkey of the newest US fighter jet, the F-35, if the Turks go through with their plan to buy Russia's advanced S-400 system instead of the American Patriot system.