NATO will experience a more fragmented, uncertain Europe with Britain outside the EU, the alliance's chief has warned, but London has given assurances that it remains committed to upholding Western stability.
With failing states on Europe's frontiers and a more aggressive Russia, NATO and the European Union are seeking to work more closely to shore up collective security.
Britain, one of Europe's biggest military powers, was meant to act as bridge between the EU and the US-led alliance.
"It is a more unpredictable situation than before the UK decided to leave. I think that's quite obvious," NATO Secretary-General Jens Stoltenberg said on Friday at alliance headquarters in Brussels.
"I am concerned about a more fragmented Europe," he told a small group of reporters.
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But Stoltenberg said British Defence Secretary Michael Fallon reassured him by telephone on Friday that Britain would not jeopardise joint EU-NATO efforts to counter potential Russian cyber attacks or naval operations in the Mediterranean, which are helping to stem an influx of migrants into Europe.
Britain will also stick to its promise to help lead the new multinational NATO force in Europe to deter Russia, Stoltenberg said.
He added that even though Britons voted on Thursday to leave the EU, NATO and the European Union will still sign a cooperation pact at the alliance's Warsaw summit in July.
Stoltenberg, a former Norwegian premier whose country has twice voted against EU membership, said he would discuss the issue with EU leaders at a summit in Brussels on Tuesday that British Prime Minister David Cameron is to attend.
He said the experience of Norway showed that cooperation between the EU and NATO remains possible.
That is despite Turkey, which is a member of NATO but not of the EU, and blocks the sharing of alliance intelligence with the European Union.
