NATO's public image in the United States and France worsened sharply last year, according to a Pew Research Center study, after US President Donald Trump and French President Emmanuel Macron questioned the value of the Western alliance.

Positive views of the North Atlantic Treaty Organization, which relies on the commitment of its allies to collective defence, fell to 52 percent in the US last year from 64 percent in 2018, the study released on Monday said.

Overall, the study said 53 percent of people in 16 NATO members had a positive opinion of the alliance, with less than a third expressing a negative view. The alliance is seen most favourably in Poland and least liked in Turkey.

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In France, where Macron said last year the alliance was experiencing "brain death" because of a perceived failure to help resolve world conflicts, support fell to 49 percent, from 60 percent in 2017 and 71 percent in 2009. The 2018 figure was not available.

NATO diplomats have long feared that Trump's portrayal of NATO as an alliance in crisis might erode US public support.

NATO, founded in 1949 to contain a military threat from the Soviet Union, relies on US military superiority to face down a host of threats on Europe's borders, including a resurgent, nuclear-armed Russia and militant attacks.

While Macron's comments stunned other NATO leaders on the eve of a December summit in the United Kingdom, Trump has harangued allies since taking office in 2017 over perceived under-spending on defence.

Trump threatened to pull America out of the alliance in 2018 and last year called low-spending allies "delinquent".

The summit in the UK revealed deep divisions within the alliance, but NATO Secretary General Jens Stoltenberg attempted to put on a united front, saying: "Whatever our differences, we will continue to unite to defend each other, all for one and one for all."

The Pew study noted that several countries "have soured on the alliance", including Germany, where support for NATO fell to 57 percent in 2019, from 63 percent in 2018.

Macron has defended his remarks as a useful wake-up call for allies, who he said were too focused on defence spending and other internal issues, rather than relations with Russia, NATO member Turkey in Syria and the Middle East.

In the UK, however, where NATO is taking on greater symbolic importance following the country's decision to leave the European Union, favourable views of NATO improved to 65 percent of Britons last year from 62 percent in 2017.