NATO is strengthening its military footprint along its eastern border immediately in response to Russia's aggression in Ukraine, the alliance's chief said Wednesday.

Secretary General Anders Fogh Rasmussen said NATO's air policing aircraft will fly more sorties over the Baltic region and allied ships will deploy to the Baltic Sea, the eastern Mediterranean and elsewhere if needed.

"We will have more planes in the air, mores ships on the water and more readiness on the land," Fogh Rasmussen told reporters in Brussels, declining to give exact troop figures.

Moscow must make clear "it doesn't support the violent actions of well-armed militias or pro-Russian separatists" in eastern Ukraine, he added.

NATO's eastern members — including Lithuania, Estonia, Latvia and Poland — have been wary following Russia's annexation of Ukraine's Crimean Peninsula, demanding a more robust military deterrence to counter neighboring Russia.

He said the NATO deployments are about "deterrence and de-escalation" in the face of Russia's aggressive behavior.

The NATO chief did not mention naval deployments to the Black Sea — which Russia would likely see as a direct aggression even though NATO members Bulgaria, Romania and Turkey also border the sea. He insisted, however, that "more will follow if needed."

NATO estimates Russia has amassed some 40,000 troops on Ukraine's eastern border and could invade parts of the country within days if it wished. Fogh Rasmussen urged Russia to pull those troops back.

The 28-nation alliance has already suspended most cooperation and talks with Russia. The United States has dispatched fighter planes to Poland and the Baltics, enabling NATO to reinforce air patrols on its eastern border. NATO also performs daily AWACs surveillance flights over Poland and Romania.