US Vice President Joe Biden promised unwavering commitment to NATO's collective defence Tuesday as he sought to reassure Baltic allies spooked by a resurgent Russia and White House hopeful Donald Trump.

"Let there be no doubt -- America's Article 5 commitment is rock solid and unwavering," Biden said, referring to a cornerstone of NATO that an attack on one member state is considered an attack on all.

Trump, the Republican presidential nominee, sparked ire last month among Baltic and eastern NATO members ruled by Moscow during the Soviet era when he questioned the alliance's key Article 5 collective security guarantee.

"Don't listen to that other fellow -- he knows not of what he speaks. America will never fail to defend our allies. We will respond," Biden said, after talks with the presidents of Latvia, Estonia and Lithuania.

"And we want you to know, we want Moscow to know, that we mean what we say."

Tensions between the 28-member NATO and Russia have reached their worst level since the Cold War following Moscow's 2014 annexation of the Crimean peninsula from Ukraine.

Asked in July by the New York Times about the Russian activities that have alarmed Baltic NATO members, Trump said that if Russia attacked them, he would decide whether to come to their aid only after reviewing whether those nations "have fulfilled their obligations to us."

"If they fulfil their obligations to us, the answer is yes," he said.

Biden said Tuesday he doubted whether Trump "even understands" what NATO's Article 5 commitment to collective defence means, while insisting it was America's "sacred honour" to fulfil it.

- Enhanced US presence -

Several of NATO's eastern European leaders have fired back at Trump, with Czech Prime Minister Bohuslav Sobotka going so far as to say that Democratic presidential hopeful Hillary Clinton is better than Trump on security for NATO allies.

"Considering the pre-election statements made by Donald Trump and Hillary Clinton, from the point of view of guarantees for the allies, I see more security if Clinton is elected," Sobotka told the Tuesday edition of Czech financial daily Hospodarskie Noviny (HN).

Clinton, a former US secretary of state, offers "a greater guarantee of maintaining trans-Atlantic cooperation," he added.

Lithuanian President Dalia Grybauskaite said Tuesday that Baltic leaders "are sure that no matter what... will be after the elections in the United States, the commitment of this country (the US) to NATO and to Baltic regions, will stay."

Hungary's anti-migrant rightwing Prime Minister Viktor Orban however became the first European leader to endorse the like-minded Trump last month, calling him a "better option" for the bloc than Clinton.

Referring to NATO's July decision to deploy multi-national battalions led by Canada, Germany, Britain and the US in the Baltic states and Poland, Biden said "the concern that the United States is somehow disengaging from the region is simply not true.

"As a matter of fact our presence will be enhanced. The fact is there will be more NATO partners on the Russian border."

Biden travels next to NATO member Turkey for high-level talks with leaders there who have been leaning toward Moscow in the wake of a failed coup attempt in July.

Biden and the Baltic leaders "expressed strong solidarity with the people and democratically-elected leadership of Turkey," in a statement issued following Tuesday talks.

The US vice president also condemned a recent string of deadly terror attacks in eastern Turkey.

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THE NEW YORK TIMES COMPANY