US President Barack Obama has called Nato "the linchpin" of US security policy and a critical ally in the fight against terrorism.
The comment indirectly counters Republican Donald Trump's recent claims that the 67-year-old alliance is obsolete.
Speaking after an Oval Office meeting with Nato secretary general Jens Stoltenberg, Mr Obama praised the alliance's contributions to the fight against the Islamic State group, its partnership in Afghanistan and assistance in the refugee crisis in southern Europe.
"Nato continues to be the linchpin, the cornerstone of our collective defence and US security policy," Mr Obama said.
Mr Obama did not name Mr Trump and ignored a reporter's question about the Republican presidential candidate's recent statements.
Still, his comments struck a strong contrast to Mr Trump's assertion that Nato is irrelevant and ill-suited to fight terrorism. As president, Mr Trump has said he would force member nations to increase their contributions, even if that risked breaking up the alliance.
Both Mr Obama and the secretary general aimed to dispute that characterisation. In remarks after the meeting, Mr Obama described Europe as especially burdened by instability and reliant on Trans-Atlantic alliance.
"This is obviously a tumultuous time in the world. Europe is a focal point of a lot of these stresses and strains in the global security system," he said. "It is because of the strength of Nato ... that I'm confident that despite these choppy waters we will be able to continue to underscore and underwrite the peace and security and prosperity that has been a hallmark of the trans-Atlantic relationship."
Mr Stoltenberg described the alliance "as important as ever".
"Nato has been able to adapt to a more dangerous world," he said, noting it began training Iraqi soldiers last week.
Mr Obama said he and Mr Stoltenberg discussed Nato's potential role in Libya, as well as its plans to assist the European Union with the migrant crisis stemming from Syria's civil war. He said the migrants are taking very dangerous trips and that the response must be "humane and thoughtful".
Mr Obama noted he has proposed quadrupling Pentagon spending on troops and training in Europe, as part of the US military's accelerating effort to deter Russia.