WARSAW — Vice President Joseph R. Biden Jr., declaring that “it’s more important today than ever that friends stand with one another,” promised Poland and the Baltic states on Tuesday that the United States would protect them from any Russian aggression similar to what has taken place in Crimea.
In the first of two days of meetings with the leaders of Poland, Estonia, Latvia, and Lithuania, Mr. Biden offered strong words and modest military and economic aid to shore up the NATO alliance and build solidarity in the face of the deepening crisis in Ukraine.
His first session, with Prime Minister Donald Tusk of Poland, occurred at the same time that President Vladimir V. Putin of Russia was making his case for annexing Crimea before the Duma. Mr. Biden and Mr. Tusk emerged, grim-faced, to speak to a phalanx of reporters.
“We join Poland and the international community in condemning the continuing assault on Ukraine’s sovereignty and territorial integrity,” Mr. Biden said as Mr. Tusk nodded.
“Russia has offered a variety of arguments to justify what is nothing more than a land grab,” the vice president continued.
“But the world has rejected those arguments,” he said, and Russia stands alone, “naked before the world” in its aggression.
Mr. Tusk was even more impassioned. With Russia’s move to annex Crimea, he said, “before our eyes, the history of this region is changing.”
It “changes overnight the borders of states,” he added. “It changes the geopolitical situation in this part of the world.”
In such turbulent circumstances, Mr. Tusk said, the alliance between NATO members and the United States looms all the larger. “Only Euro-Atlantic solidarity will allow us to prepare sufficient and strong reactions to Russia’s aggression,” he said.
Mr. Biden’s trip is designed to send a visible message to Russia, reinforcing the sanctions the United States and the European Union announced Monday against Russian officials and their allies for their role in the referendum in the breakaway peninsula of Crimea.
But it also comes at a time of deep uneasiness in Eastern Europe about the reliability of the United States as a guarantor of its security. President Obama’s so-called pivot to Asia was viewed by many here as a turning-away from Europe, while NATO has seemed to be groping for new purpose in the long twilight of the war in Afghanistan.
Poland and the Baltics are less vulnerable than Ukraine because their NATO membership guarantees they will be defended if they come under attack, but their history as Soviet satellites has left deep anxieties that have been revived by the Russian incursion into Ukraine.
In Poland, Ukraine has become a volatile political issue, with the opposition sounding alarms that this country could be next for Russia and criticizing Mr. Tusk’s government, which had cultivated constructive relations with Moscow.
To calm the waters, the vice president announced increased technical assistance to help modernize the Polish armed forces and additional joint military training. The United States has already sent an additional 12 F-16 fighter jets to an aviation detachment in Poland, as well as 10 more F-15’s to a NATO operation that polices the skies over the Baltic States.
But Mr. Biden announced no changes to the American missile defense system in Poland and Romania as a result of the crisis. He said it was on track and would be operational by 2018.
The vice president also discussed energy security with the Europeans, many of whom rely on Russia for more than a third of their natural gas imports. He urged them to diversify their energy sources by investing in shale and nuclear technology, making them less vulnerable to Russia using its gas shipments as a political weapon.
Lawmakers have called on the Obama administration to speed up permits that would allow the United States to accelerate shipments of liquefied natural gas to Europe. But converting the gas for shipment is expensive, and to date, the administration has issued only six permits for the construction of export terminals for liquefied natural gas.
The vice president has played a substantial role throughout the Ukraine crisis, initially in talking to the country’s ousted president, Viktor F. Yanukovych, and more recently in meeting with other neighbors threatened by Russia: Georgia and Moldova. Mr. Obama has also stepped up his engagement, speaking recently to Prime Minister Tusk.
He interrupted a recent golf weekend in Florida to hold a conference call with the three Baltic leaders: President Andris Berzins of Latvia, President Dalia Grybauskaite of Lithuania, and President Toomas Hendrick Ilves of Estonia. He told them the United States had an “unwavering commitment” to their defense, according to the White House.
In Warsaw, in addition to Mr. Tusk, Mr. Biden will meet President Bronislaw Komorowski and Mr. Ilves, who happens to be on a state visit to Poland. In the Lithuanian capital, Vilnius, on Wednesday, he will meet Ms. Grybauskaite and Mr. Berzins.