The United States will review its troop presence in Europe as it seeks to reassure allies they will not be left unprotected after Russia's intervention in Ukraine, US Defense Secretary Chuck Hagel said Wednesday.

President Barack Obama's $1 billion US 'reassurance plan' for eastern Europe announced Tuesday is a key part of such efforts and will involve Washington "reviewing the US force presence in Europe," Hagel said.

"In light of the new regional security environment, it would be irresponsible for us not to," he added, in an apparent reference to Ukraine.

This is a sensitive issue at the heart of the NATO-Russia treaty that formalised the end of the Cold War and set the parameters for security in Europe with Moscow.

The 1997 NATO-Russia Founding Act said the West, led by the United States, would not permanently deploy troops or arms, including nuclear weapons, in former communist states ruled by Moscow in eastern Europe.

Both sides also agreed that neither should treat the other as an "adversary", within a new cooperative security framework.

But Russia has warned that recent NATO rotational deployments of additional planes and troops in states such as Poland and the Baltic countries -- described as a gesture of reassurance -- amounts instead to a breach of the treaty.

NATO head Anders Fogh Rasmussen insisted Tuesday that NATO's response was entirely within the limits of the Founding Act and it was Russia that was in "blatant breach" of the treaty for its actions in Ukraine.

Hagel said all NATO member states must do everything possible to bolster and strengthen the alliance, including finding the extra money needed to meet the new threat posed by Russia.

Defence budgets have fallen in Europe, meaning the United States has taken on a disproportionate burden and there would be a "threat to NATO capabilities unless we reverse" these trends, he said.

This will be a key issue for a NATO leaders summit in Britain in September, he added.

Rasmussen, speaking before Hagel, made similar points about the alliance's future and noted that Georgia was making "real progress" towards NATO membership.

Georgia, which was offered NATO membership in 2008 along with Ukraine, has pressed ahead with the democratic and other reforms required of member states, he said.

While "more remains to be done, Georgia has made real progress ... and is on the right track," he said.

NATO leaders agreed at a 2008 summit shortly after Georgia fought a brief war with Russia that it could join the military alliance but only at an unspecified future date.