Alarm and condemnation erupted from European capitals, the White House and leaders of Donald Trump's own party after the Republican presidential nominee suggested the United States might abandon its Nato military commitments if he were elected president.

Senate Republican leader Mitch McConnell, who backed Mr Trump at the party's national convention only two days earlier, said he totally disagreed with the statement but was willing to "chalk it up to a rookie mistake".

Mr McConnell called Nato "the most successful military alliance in the history of the world", in a Facebook interview with The New York Times.

In Brussels, Nato secretary-general Jens Stoltenberg said the alliance agreement was crystal clear: "We defend each other."

"I will not interfere in the US election campaign," Mr Stoltenberg said. But he pointedly added: "Two world wars have shown that peace in Europe is also important for the security of the United States."

White House spokesman Josh Earnest noted that every president since the Second World War, Republican and Democratic, has supported the Nato agreement.

"The cornerstone of that alliance is the pledge that all of the allies have made to mutual self-defence," the White House spokesman said. "The US commitment to that pledge is ironclad."

Indeed, Mr Trump's suggestion, in an interview with the Times, would upend decades of American foreign policy and rock the security structures that have underpinned European and global stability since the end of the Second World War.

Mr Trump said in the Times interview that he would review allies' financial contributions - in this case, those from Estonia, Latvia and Lithuania - before acting under Nato's mutual defence clause, if any of the countries were attacked by Russia.

Various US administrations have complained, often bitterly, that many Nato members do not foot their share of the alliance's bills.

The US accounts for more than 70% of all Nato defence spending and only four other allies - Britain, Estonia, Greece and Poland - meet the minimum 2% of gross domestic product spending on defence that Nato requires.

But Mr Trump's floating of the idea that the spending target would be a prerequisite for the US to defend a Nato ally was an abrupt break from longstanding American policy.

Estonian president Toomas Hendrik Ilves tweeted that his country was one of the few to meet the minimum defence expenditure and noted pointedly that Estonia "fought, with no caveats" on behalf of the US in Afghanistan.

The only time the treaty's mutual defence clause has been invoked was in 2002, when Nato surveillance planes patrolled American skies and deployed a third of the troops sent to Afghanistan for a decade. More than 1,000 non-American troops died in Afghanistan.

Mr Ilves' fellow Eastern European leaders sought to calm the furore.

"Regardless of who will be the president of America, we will trust in America," Lithuanian president Dalia Grybauskaite told reporters in Vilnius.

Yet people throughout Eastern Europe expressed deep concern. Fears of Russian aggression have run high since it annexed the Ukrainian region of Crimea.

"His words were irresponsible and they inspired fear in me. I'm worried about the world's future, about Poland's future," said 39-year-old school teacher Lidia Zagorowska in Warsaw, Poland.

"If I were a US citizen I would never, ever vote for Trump. Let that be my answer," said Katarzyna Woznicka, 54, walking her dog in central Warsaw.

Back in the United States criticism, including some from Mr Trump's fellow Republicans, was blistering.

"My hope is that if Donald is elected president, we can convince him to change his mind on it," said Senator Marco Rubio, a former primary opponent who now supports Mr Trump.

A bitter foe within Mr Trump's own party, Senator Lindsey Graham of South Carolina, said: "I'm 100% certain how Russian president (Vladimir) Putin feels - he's a very happy man."

Some Republicans opposed to Mr Trump have indeed sought to cast him as pro-Putin, a position that would put him at odds with both Republican and Democratic foreign policy and also diverge from the current Republican party platform adopted at the convention.

Trump supporters succeeded in preventing a reference to arming Ukraine from getting into this year's platform, but the manifesto itself is demonstrably not pro-Russia. It accuses "current officials in the Kremlin" of eroding the "personal liberty and fundamental rights" of the Russian people.

"We will meet the return of Russian belligerence with the same resolve that led to the collapse of the Soviet Union," the Republican platform says.

"We will not accept any territorial change in Eastern Europe imposed by force, in Ukraine, Georgia, or elsewhere, and will use all appropriate constitutional measures to bring to justice the practitioners of aggression and assassination."