NATO said Thursday there have been around 400 intercepts of Russian military flights near its member countries this year, amid heightened tension between Moscow and the West over the Ukraine crisis.

"If you look at the number of intercepts around NATO, we can talk about 400 intercepts, 50 percent more than last year," NATO Secretary General Jens Stoltenberg told reporters in western Estonia.

"Most of these flights are taking place in an international airspace, but they are close to our airspace, and they are interfering with commercial flights.

"It is a pattern that we haven't seen for many years, back to the time of the Cold War."

He spoke from the Amari air base that hosts the Western defence alliance's air policy mission over the Baltic states, following talks with Estonian Prime Minister Taavi Roivas.

It was his first stop on a tour of the three Baltic states that were once ruled from Moscow and which, like fellow newer NATO member Poland, have been deeply concerned by Russia's actions in Ukraine.

The West believes Russia is pulling the strings in the deadly seven-month conflict between pro-Western government forces and pro-Russian separatists in Ukraine's east -- an accusation the Kremlin denies.

Stoltenberg said NATO had seen "more than 100 intercepts, which is three times more than last year" around the Baltic region alone, but reiterated that the alliance would protect its members.

"It is all for one and one for all," he said.

He later travelled to Latvian capital Riga, where he reacted to Russia's Wednesday call for Ukraine to stay out of the NATO alliance.

Requiring Kiev to give such a guarantee violates "the idea of respecting the independence, the sovereignty of Ukraine," he said after talks with Latvian President Andris Berzins.

"Each and every country has the right to decide its own security arrangements."

He said he expects Russia to "respect" the decision of Ukraine were it to apply for NATO membership "later on".