Labour splits on defence have resurfaced after Jeremy Corbyn's team suggested Britain's deployment of troops and jets to Estonia was "escalating tensions" with Russia.
A source close to shadow defence secretary Nia Griffith said she was "absolutely furious" after Mr Corbyn's spokesman appeared to question the Nato mission designed to deter Russian aggression.
The spokesman said the leader had "expressed concerns" about the deployment of hundreds of troops to the Baltic state, where RAF Typhoon fighter jets have also been involved in the interception of Russian warplanes.
The source said Ms Griffith's support for Nato is a "red line" but she was not considering her position in the shadow cabinet.
The row broke out after Russia accused Britain of preparing an "official witch-hunt" against President Vladimir Putin's regime.
Questioned about the attack on Wednesday, Mr Corbyn's spokesman said the leader had repeatedly voiced criticisms of the Russian government over its actions in the Middle East and domestically.
But he added: "What we don't want to see is a ratcheting up of tensions between Russia and the West, as has been taking place. We want to see an engagement with Russia - on a critical basis, but a serious basis."
The spokesman said Mr Corbyn was worried about tensions on the border between Russia and the Baltic nations.
"We've said repeatedly that we want talks and engagement to wind down military tensions, particularly on the Nato/Russian border and in the Middle East," said the source.
"Jeremy has expressed concerns about that being one of the escalations of tensions that have taken place."
It comes as Labour divisions over energy policy are set to come to the fore in a by-election in Copeland, where the local economy relies heavily on Sellafield.
Mr Corbyn has been a long-standing opponent of new nuclear power plants and in a recording from 2011 said he wants to decommission existing stations.
"No nuclear power, decommission the stations we have got and let's have an intelligent energy policy based on sustainability, not destruction," he said.
Local Labour councillor Gillian Troughton, who is vying to become the party's candidate in the by-election, distanced herself from the party leader.
She said: "It's quite clear that Labour policy is for new nuclear.
"Jeremy Corbyn is not the entire Labour Party and Labour policy is for the green, low carbon energy policy of which nuclear power is a key part."