US Secretary of State John Kerry has said the Obama administration remained committed to the NATO military alliance and security commitments under the treaty.
He was responding to comments by Republican presidential nominee Donald Trump, who told the New York Times that if elected he might abandon NATO's guarantee that any member, including the United States, would defend the others if they were attacked.
Mr Kerry said "I want our NATO partner to be clear where we stand. This administration, like every single administration Republican or Democratic alike since 1949, remains fully committed to the NATO alliance and to our security commitments on the Article 5".
Mr Trump has raised fresh questions about his commitment to automatically defend NATO allies if they were attacked, a stance in keeping with his "America First" agenda.
In response to a question about potential Russian aggression towards the Baltic states, Mr Trump told the newspaper in an interview that if Moscow attacked them, he would decide whether to come to their aid only after reviewing whether those nations"have fulfilled their obligations to us".
He added: "If they fulfil their obligations to us, the answer is yes."
Mr Trump was quoted as saying he would force allies to shoulder defence costs that the United States has borne for decades, cancel longstanding treaties he views as unfavourable, and redefine what it means to be a partner of the US.
"I would prefer to be able to continue" existing agreements, he said, but only if allies stopped taking advantage of what he called an era of American largesse, the New York Times wrote.
Democratic presidential candidate Hillary Clinton's senior foreign policy advisor, Jake Sullivan, said in a statement: "Trump has apparently decided that America lacks the moral authority to advance our interests and values around the world."
David Corn, Washington bureau chief of the Mother Jones news website, said in a tweet that Mr Trump's campaign manager Paul Manafort had told him the candidate had been misquoted.
Manafort tells me NYT misquoted Trump on defending NATO allies and respecting existing agreements. Says it does this a lot. NYT, got a tape?
— David Corn (@DavidCornDC) July 21, 2016
Reuters was unsuccessful in attempts to reach Trump campaign staff for comment.
Mr Trump has for months raised questions about the money the US pours into NATO, which he says needs to be reconfigured to take account of today's global threats.
His rhetoric has raised alarm in allied countries that still rely on the US defence umbrella.
The phrase "America First" was used in the 1930s by isolationists who sought to keep the US out of World War II.
Mr Trump was quoted as saying that if elected, he would not exert pressure on Turkey or other authoritarian allies about purging political opponents or cracking down on civil liberties.
He said the US has to "fix our own mess" before trying to sway the behaviour of other nations.
"I don't think we have a right to lecture," Mr Trump was quoted as saying.
"Look at what is happening in our country," he added. "How are we going to lecture when people are shooting policemen in cold blood?"
Yesterday Mr Trump was officially declared the Republican's presidential nominee at the party's national convention in Cleveland.
The gathering has been mired in controversy since it began as it emerged that parts of Melania Trump's speech was plagiarised from Michelle Obama's speech to the Democratic convention in 2008 and also as former Trump rival Ted Cruz failed to nominate Mr Trump in his speech.
China slams US Republicans for 'groundless accusations'
China has warned the Trump-fronted US Republican Party against making "groundless accusations" after it accused Beijing of cultural genocide and said its economy was based on piracy.
The GOP said in its 2016 platform that Beijing made "a preposterous claim to the entire South China Sea", adding that "cultural genocide continues" in the regions of Tibet and Xinjiang.
"The currency is manipulated, our technology is stolen and intellectual property and copyrights are mocked in an economy based on piracy," it added.
Chinese foreign ministry spokesman Lu Kang called on the party to "stop their groundless accusations against China and interference in China's domestic affairs", in a statement carried by the official Xinhua news agency.
"All political parties in the United States should view China's development in an objective and rational manner," he added, in Beijing's first comments on the Republican Party since Mr Trump's nomination as its presidential candidate yesterday.
The US and China are the world's two largest economies with deep trade ties, but tensions remain over security and human rights issues.
China has been seen as sceptical of Democratic candidate Hillary Clinton, a consistent critic of Beijing's rights record who last year slammed its detention of feminist activists as "shameless".
As secretary of state, Mrs Clinton also oversaw the US "pivot" to Asia, which many in China see as an effort to contain Beijing's rising influence in the region.
US politicians have long criticised China's human rights record, taking Beijing to task for its treatment of ethnic minorities in the mostly Muslim, western region of Xinjiang and in Tibet.
Economic and trade issues have also been a frequent point of discord. The US government under President Barack Obama has censured Beijing for overlooking intellectual property theft from American companies and the manufacture of pirate goods.
The Republican Party has often received a warmer reception in Beijing, seen as more focused on trade and with historical ties to Richard Nixon, who engineered a detente between the two powers.
But Mr Trump has also made China-bashing a staple of his campaign, saying the country was "raping" the US through trade and calling for tariffs of 45% on Chinese goods.
China's finance minister Lou Jiwei reportedly called Mr Trump an "irrational type" in April, suggesting Beijing was uncomfortable with the Republican's tone under his leadership.
