If Britain votes to leave the European Union it could have a negative impact on the Nato alliance, a senior US military commander has warned.
Lt-Gen Ben Hodges, head of the US Army in Europe, said he was "worried" the EU could unravel just when it needed to stand up to Russia.
He acknowledged the vote was a matter for the British people, but said he was concerned about the outcome.
Out campaigners say a leave vote would not affect the UK's position in Nato.
The UK has been a member of the transatlantic defence alliance since its formation in 1949. It joined the European Economic Community, the forerunner to the European Union, in 1973.
The UK will vote on whether it should remain a member of the EU in a referendum on 23 June.
General Hodges told the BBC that Russian aggression in Ukraine and its intervention in Syria had threatened Europe's security.
Russia had "weaponised" the migrant crisis by forcing tens of thousands of people to flee to Europe, he said.
The Nato alliance and the EU needed to show solidarity in the face of Russian aggression, but that unity could be put at risk if Britain voted to leave the EU, he claimed.
"The UK is such an important member of the alliance," he said. "It is a leader in the alliance. It is a leader in Europe. The most reliable trusted friends and allies we have are all European countries and so what goes on here is of strategic interest to us
"Anything that undermines the effectiveness of the alliance has an impact on us, and so if the EU begins to become unravelled there can't help but be a knock-on effect for the alliance also."
Last month, Nato secretary general Jens Stoltenberg suggested the organisation would prefer a strong Britain within the EU, 22 of whose members are also part of Nato.
The commander's comments come amid a debate about the right of foreign leaders to speak out over Britain's future in the EU.
US President Barack Obama is expected to signal his support for the UK's continued EU membership during a visit to London next month and speaking at a campaign event in Felixstowe, Prime Minister David Cameron said he didn't know a "friendly foreign leader" who supported the UK leaving the EU.
But Conservative MP Tom Pursglove, a co-founder of the Grassroots Out campaign group, said the EU had "very little to do with our security arrangements" and the UK would remain a "pivotal" member of Nato if it left, as well as a member of the G7, G20 and UN.
"I find it incredibly unhelpful when we hear American figures and figures from all over the world telling the British people what to do," he told the BBC News Channel. "We know President Obama has a very strong view on these matters.
"I wouldn't seek to tell the US electorate who to vote for - I wouldn't say whether they should vote for Donald Trump, Marco Rubio or Hillary Clinton....I think this is a decision for the British people and the British people alone."
Russia has criticised the UK government for claiming that its President Vladimir Putin wants the UK to secede from the EU in order to weaken the West, insisting that it has "no opinion" on the subject.