At a meeting with journalists from the Baltic states at the NATO Headquarters in Brussels, the NATO chief recalled the Cold War era when t he war of nerves with Russia had peaked.

"Deterrence is the key, the unity of NATO is the key. All countries are different and all situations are different but West Berlin during the Cold War was the island in the middle of East Germany. But they felt safe because they knew that NATO was behind and sending a clear message about that NATO will protect West Berlin," said Stoltenberg.

"I am from Norway. And Norway has been very close to the all the strategic forces, the nuclear forces, the submarines of Kola peninsula. (…) There was no big NATO forces in the Norway but we felt very safe because we knew that if there is a danger, that if there is a crisis, NATO will be here to protect us," he added.

"This is why we working with the Baltic countries," said the NATO secretary general.

The Alliance's concerns about the Moscow actions were fueled by Wednesday's deployment of two vessels armed with nuclear-capable Kalibr long-range missile systems in the Russian Baltic Fleet.

The West is also concerned about Russia's actions in its Baltic Sea exclave of Kaliningrad where Moscow several weeks ago stationed nuclear-capable Iskander missile systems as part of military drills.

The Alliance said it was keeping an eye on the heightened Russian military activity in the Baltic Sea region but said there would be no immediate response.

Stoltenberg called for attention to the broad picture of Russia's military activity, including the aggression in Ukraine and Syria, constantly growing capacities and frequent exercises, which are not always announced in advance.

"We are not mirroring what Russia is doing but we will respond in a measured and responsible way," said the head of the Alliance.