Republican presidential nominee Donald 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, the New York Times reported.

In response to a question about potential Russian aggression towards the Baltic states, 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."

Trump was quoted as saying he would force allies to shoulder defense 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 United States.

"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 largess, the New York Times wrote.

Democratic presidential candidate Hillary Clinton's senior foreign policy adviser, 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 Trump's campaign manager Paul Manafort had told him the candidate had been misquoted.

Reuters was unsuccessful in attempts to reach Trump campaign staff for comment late on Wednesday.

NATO says solidarity a 'key value'

NATO Secretary General Jens Stoltenberg said he wouldn't comment on the U.S. election, but he did release a statement saying 'peace in Europe is also important for the security of the United States.' (Darko Vojinovic/Associated Press)

Jens Stoltenberg, NATO's Secretary General, said he would not interfere with the U.S. election campaign, but said he would comment on "what matters for NATO."

He said that solidarity among NATO's allies is a "key value" that is good for European and American security.

"The United States has always stood by its European Allies. Now the U.S. is stepping up its support once again, and increasing its presence," said Stoltenberg, who added that for the first time in years, European members of NATO have increased their defence spending.

"Two world wars have shown that peace in Europe is also important for the security of the United States."

Trump's choice as his running-mate, Mike Pence, says he's confident that Trump would stand by the nation's NATO allies, but insisted those countries "must pay their fair share."

Pence told Fox News that a Trump administration would tell U.S. allies "the time has come for them and for their citizens to begin to carry the financial costs of these international obligations."

Trump rhetoric raised alarm

Trump has for months raised questions about the money the United States 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 U.S. defense umbrella.

The phrase "America First" was used in the 1930s by isolationists who sought to keep the United States out of World War Two.

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 United States has to "fix our own mess" before trying to sway the behavior of other nations.

"I don't think we have a right to lecture,", 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?"