A Union Jack flag flutters next to European Union flags ahead of a visit from Britain's Prime Minister David Cameron at the EU Commission headquarters in Brussels, Belgium February 16, 2016. REUTERS/Yves Herman

LONDON (Reuters) - A British exit from the European Union could trigger the unraveling of the bloc and thus weaken the NATO military alliance in relation to Russia, the head of the U.S. Army in Europe was quoted as saying by the BBC on Tuesday.

Prime Minister David Cameron has called a June 23 referendum on Britain’s membership of the world’s biggest trading bloc. Opinion polls show British voters are divided with a large number of people still undecided.

“Anything that undermines the effectiveness of the alliance has an impact on us, and so if the EU begins to become unraveled there can’t help but be a knock-on effect for the alliance also,” Lieutenant General Ben Hodges told the BBC.

Hodges, who was commander of NATO’s Allied Land Command from 2012 to 2014, said the vote was a matter for the British people but said he was concerned about the impact on the NATO alliance.

“What goes on here is of strategic interest to us,” he said, adding that NATO and the EU needed to show solidarity in what he described as Russian aggression.

Russia had “weaponized” the migrant crisis by forcing tens of thousands of people to flee to Europe, he was quoted as saying by the BBC.

U.S. President Barack Obama will come to London in April and urge British voters to back membership of the European Union, The Independent on Sunday newspaper reported. [L5N16K0QA]

Russia has scolded Britain for saying that Kremlin chief Vladimir Putin dreams of weakening the West with a British exit from the EU. [L5N16J49F]