The interview came as Turkey threatened to block new NATO plans to defend the Baltic states and Poland -- presumably from a Russian attack -- unless Western powers recognised a Kurdish militant group as terrorists.

"Through the presence of NATO forces in Poland and the Baltic states we are sending a very strong signal to Russia: if there is any attack on Poland and the Baltic states, the entire alliance will respond," Stoltenberg told Poland's Rzeczpospolita newspaper as the alliance met for a summit riven by tensions in London.

"We do not define Russia in this way (as an enemy)," Stoltenberg said, adding that "we must be certain that there is no possibility that what we saw in Ukraine, meaning Russia's armed invasion of its neighbour, could be repeated against a NATO member".

President Recep Tayyip Erdogan vowed on Tuesday that Turkey would block a mooted plan to defend Baltic countries if the allianced failed to give Ankara support in its fight against the Syrian Kurdish People's Protection Units (YPG).

The NATO summit, marking the alliance's 70th anniversary, was set to be a tense affair with Turkey at odds with other members over its purchase of Russian missiles and recent offensive in northern Syria.

US President Donald Trump also declared on Tuesday that his French counterpart Emmanuel Macron's criticism of NATO as "brain dead" was very insulting.

"NATO serves a great purpose," Trump said, at a joint press appearance with Stoltenberg ahead of the summit.