Russia's defence minister has said Russia will not tolerate the "encroachment" of Nato military infrastructure on Russia's borders.
By Steven Eke
BBC Russia analyst
Sergei Ivanov said any such moves would cause the Kremlin "very obvious concern".
Despite a good working relationship between Russia and Nato, the idea of Nato's expansion eastwards remains highly unpopular in Russia.
But Russia has little scope for response.
Just a few years ago, an announcement of possible Nato expansion eastwards would have caused outrage in Russia.
Yet in his first response, Defence Minister Ivanov merely said the security of some must not come at the expense of others.
The muted Russian response reflects both the vagueness of the American proposals as well as the much improved relationship Nato and Russia enjoy.
Far from the angry rhetoric that once characterised Russian policy, the Kremlin now speaks with satisfaction of the joint programmes, high-level consultations and intelligence sharing in the US-led war on terror.
Russian military forces face low morale
That said, for many Russians - officers and politicians, as well as ordinary people - Nato's expansion east-wards is a potent reminder of who lost the Cold War.
And the idea of American bases in the countries of the former socialist bloc is salt in slowly-healing wounds.
The reality is that Russia has little or no scope for a military response.
Its armed forces are too weak.
And, quietly tolerated by the Kremlin, there has already been a steady expansion of Western military activity in several parts of the former Soviet Union - including a now significant presence in Central Asia.
However, the announcement comes at a sensitive time and will cause political upset.
Russia is irritated by what it sees as American interference in the Caucasus.
It is also angry at being barred from bidding for reconstruction contracts in Iraq.
And recent elections saw a solid core of nationalists elected to parliament.
Wednesday's announcement is likely to compound the already widespread anti-American feeling in Russian society.