US Vice-President Mike Pence has reaffirmed Washington's commitment to the security of the Baltic states if they face any aggression from Russia.
Speaking in Estonia, he called Russia their biggest security threat, telling the Nato allies: "An attack on one of us is an attack on us all."
The remarks come amid growing tensions as Russia orders a cut in US diplomatic staff numbers.
The move was in retaliation for new US sanctions against Russia.
The Baltic states - Estonia, Lithuania and Latvia - all achieved independence as a result of the break-up of the Soviet Union.
They have grown increasingly wary of Russia since it annexed the Ukrainian peninsula of Crimea in 2014, and count on Nato allies coming to their aid in the event of any Russian action.
There had been doubts about President Trump's commitment to the key Nato principle of mutual defence, after he did not mention it during a visit to Nato's Brussels headquarters in May and instead lambasted fellow members for not spending enough on defence.
But he re-committed the US to it during a Washington press conference in June, and more symbolically at a speech in Poland in early July.
After meeting the presidents of all three Baltic states, Mr Pence said that the message of President Donald Trump to them was: "We are with you."
"Under President Donald Trump, the United States stands firmly behind our Article 5 pledge of mutual defence," he said at a news conference in the Estonian capital, Tallinn.
"A strong and united Nato is more necessary today than at any point since the collapse of communism a quarter-century ago," he said.
"And no threat looms larger in the Baltic states than the spectre of aggression from your unpredictable neighbour to the east."
Russia is preparing to hold large-scale military exercises with its ally Belarus in the next month.
Analysis: Carving out an uncertain path
Jonathan Marcus, BBC defence and diplomatic correspondent
Russia's decision to significantly reduce the number of US diplomatic staff marks the end of any immediate hope for a fresh start between Moscow and Washington. Indeed, it could usher in a new and uncertain period of competition between the two capitals.
Russia's President Vladimir Putin seized upon US President Donald Trump's election victory as offering the potential for a very different relationship.
Mr Putin appears to have misjudged what was on offer. Relying upon clear signals from the Trump campaign that a reset in the US relationship was both possible and desirable, Moscow intruded into the US campaign to an extent which US intelligence agencies believe was unprecedented.
Quite how far Russia intruded into the US campaign in actually backing candidate Trump is currently under investigation and this, of course, provides an important element of the backdrop to the worsening diplomatic relationship.
The US is currently holding its biggest ever joint military exercises with Georgia, a country which fought a war with Russia in 2008 and seeks to join Nato.
Mr Pence is scheduled to attend the drills in Georgia on Tuesday. He will later go to Montenegro, Nato's newest member.
In Estonia, the US vice-president also warned that Moscow continued to "seek to redraw international borders by force, undermine democracies of sovereign nations and divide the free nations of Europe, one against another".
His visit to the region comes as Moscow announced that 755 staff must leave US diplomatic missions in Russia, in retaliation over new sanctions imposed for Russia's annexation of Crimea in 2014 and Russian interference in the US election.
Reacting to the move, Mr Pence said: "We hope for better days, for better relations with Russia.
"Recent diplomatic action taken by Moscow will not deter the commitment of the United States of America to our security, the security of our allies."