NATO defense ministers agreed Tuesday to a series of steps to bolster protection in eastern Europe after the Ukraine crisis, while insisting they were acting within the limits of a key post-Cold War treaty with Moscow.

NATO Secretary-General Anders Fogh Rasmussen said ministers agreed to develop a "readiness action plan ... to respond to the changed security environment" created by the escalating conflict in Ukraine.

The plan will include measures such as positioning supplies and equipment in member states and stepping up work to improve military capabilities to help NATO speed up its reaction time to any threat.

The plan will go to NATO leaders at their September summit in Britain for approval.

The decision comes after Moscow's annexation of Crimea and support for pro-Russian separatists in the east of Ukraine plunged East-West relations to their worst point since the end of the Cold War.

Russia's actions mean it "is in blatant breach of the 1997 Founding Act," Rasmussen said.

The 1997 NATO-Russia Founding Act formalized post-Cold War borders in Europe and crucially said neither the West, led by the United States, nor Russia would deploy forces or arms in the newly freed east European countries in a threatening manner.

Both sides also agreed that neither should treat the other as an "adversary," aiming to reduce the risk of future conflict.

Russia's envoy to NATO, Alexander Grush, said Monday that alliance's temporary deployment of additional alliance planes and troops in member states such as Poland and the Baltic countries amount to a breach of the treaty.

Some NATO member states, especially in central and eastern Europe, were surprise at Russia's ability to mass 40,000 troops on the border with Ukraine very quickly and keep them there, ready for action, for some time.

Rasmussen said NATO had to take necessary measures for "as long as necessary" to counter a new threat.

He pointed out that Russia had increased defense spending by 50 percent over the last five years, while the allies have cut theirs by a fifth.

Rasmussen warmly welcomed President Barack Obama's announcement of a $1 billion U.S. security plan for eastern Europe, aimed at reassuring NATO allies that have been increasingly concerned by Russian actions.