U.S. President Barack Obama has urged NATO to make "concrete commitments" to help Ukraine modernize and strengthen its security forces in the face of Russian "aggression."
Obama was speaking in the Estonian capital, Tallinn, on September 3 following talks with the leaders of Estonia, Latvia, and Lithuania.
The U.S. president, who is heading later to a NATO summit in Wales that starts on September 4, said NATO must send an "unmistakable message of support" to Ukraine.
The country has been battling pro-Russian separatists in the country’s east since April.
Kyiv and the West accuse Russia of sending troops and weapons to reinforce the rebels, which Moscow denies.
Accusing Russia of a "brazen assault" on Ukraine, Obama said the United States will continue to offer training and assistance to help strengthen Ukraine's military.
He said the alliance should also "do more to help other NATO partners, including Georgia and Moldova," strengthen their defenses.
Obama said NATO must leave the door open to new members, without making an explicit reference to any country.
Addressing concerns over Russia's possible expansion into the Baltics, where ethnic Russians make up a sizable minority, Obama said NATO would not hesitate to protect one of its members.
Referring to Estonia, Latvia, and Lithuania, Obama said: "You lost your independence once before. With NATO, you'll never lose it again."
Earlier on September 3, Obama assured Estonia of U.S. support for regional security in the Baltics and the "unbreakable" U.S. commitment to defend any NATO ally against attack.
At a news conference in Tallinn with Estonian President Toomas Hendrik Ilves, Obama also said Russia's role in the conflict in eastern Ukraine "gives us an opportunity to understand the importance" of updating and bolstering NATO's defense capabilities.
Ilves said the conflict in eastern Ukraine "is Russian aggression" and underscores the need for a "robust and visible" NATO presence in his country.
He added that "Russia must admit that it is a party to the conflict and take genuine steps for a de-escalation" of the crisis.
One day earlier, Ilves said Estonia wants NATO to set up permanent bases on its territory to protect it against potential threats from Russia.
Obama told the news conference he would seek support from Congress to deploy additional aircraft and U.S. troops on a rotating basis at Amari Air Base at Harjumaa, Estonia.
NATO already has increased the number of its aircraft in Poland and the Baltic states as the Ukraine crisis has unfolded, with Russia taking similar steps with its air forces in the region.
The Pentagon last month also announced the deployment of Abrams M-1 tanks and armored fighting vehicles in Poland and the Baltic states for what it called a three-month "training exercise."
NATO Secretary-General Anders Fogh Rasmussen has said NATO leaders at the summit in Wales would be asked to approve the creation of a high-readiness force to help the alliance's easternmost members defend themselves.
Rasmussen -- who has accused Russia of engaging in "direct military operations" in Ukraine -- said the plan calls for a "spearhead" force comprising several thousand troops that would be contributed on a rotating basis by all 28 NATO countries.
He said the plan would not breach an agreement struck with Russia in 1997 that limits NATO's troop presence in the East.
But Rasmussen also said the plan was aimed at showing "any potential aggressor" that an attack on NATO’s eastern members would target the entire alliance.
Moscow has said it will take steps to respond "with a view to ensure its security" if NATO begins regular troop rotations in the alliance’s easternmost countries.
Finland and Sweden, both members of NATO's Partnership for Peace program, have said they are signing a pact allowing them to host NATO training exercises and grant greater access to NATO troops on their territory.