The shadow defence secretary, Nia Griffith, was said by sources to be “absolutely furious” on Wednesday night after a spokesman for Jeremy Corbyn appeared to question the recent decision to send British troops to Estonia.

Briefing journalists after prime minister’s questions, in which Corbyn pressed Theresa May over the health service, the spokesman suggested both sides, Russia and Nato, had to share the blame for the recent souring of relations.

Asked whether Labour supported the government’s decision to send 800 troops to Estonia as part of a Nato taskforce, he said: “Jeremy has expressed concerns about that being one of the escalations of tensions that have taken place.”

Griffith, who has visited the troops in Estonia and had earlier used an interview with Forces TV to express staunch support for Nato, was said to be “absolutely furious” about the remarks.

In Griffith’s interview, given hours earlier, she said: “I think it’s very, very, important that we now play a very strong role in Nato, particularly as we are leaving the European union. I think it’s very important for Nato to be absolutely clear, following what has happened in Ukraine, that we are standing together as Nato nations and there is no way that we would tolerate any attack on any one of our member states.”

Usually regarded as an ally of Corbyn, Griffith was handed the post when Clive Lewis was moved to the business brief.

In the briefing Corbyn’s spokesman also said: “Jeremy has said repeatedly that he has lots of criticisms of the Russian government, both in relation to what has happened in the Middle East and domestically. But 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 talks and engagement to wind down military tensions, particularly on the Russian/Nato border and in the Middle East.”

The spokesman also appeared to reopen an argument about whether Corbyn believed Nato members were bound to offer military support if a fellow member were attacked. Asked what would happen if, for example, Russia invaded Estonia, the spokesman said the appropriate response could fall short of military action.

He said: “Article 5 [of the Nato treaty] means that if there is an attack on another Nato state there has to be a response by Nato members – it doesn’t define what that response should be. In practice, if you see from the wording of the treaty, that can be a whole range of things. But what we’re looking for is a reduction of tension and a negotiated engagement.”

Labour MP Mike Gapes, who has spoken for ‘collective defence’. Photograph: John Stilwell/PA

Some Labour backbenchers also voiced unhappiness. The Ilford South MP, Mike Gapes, tweeted: “Labour Attlee established Nato. Under Blair enlarged to Baltic States. Article 5 collective defence absolutely essential for all members.”

The fresh row came as Corbyn’s team pronounced themselves pleased with his populist new-year relaunch, and said he would continue to promise radical action to tackle inequality in the coming weeks. Corbyn had raised the prospect of legislating for a “maximum wage”, in apparently unscripted remarks during a radio interview Tuesday morning as part of a wave of media appearances. In a speech later in the day he floated other proposals, including using government contracts to encourage firms to limit the pay ratio between lowest-paid workers and chief executives.

Some Labour MPs were left confused about his stance on the issue of immigration, on which his party is deeply divided. “People are just in despair,” said one sceptical backbencher after the leader appeared to retreat from plans to stress he had no objection to immigration controls.

But Corbyn’s advisers believe the details of party policy are less important to the public than the general idea that he is ready to challenge vested interests, and they hope his strong language on fat cat pay will play well. They draw a parallel between the pay ratio plans, and Ed Miliband’s policy of a price freeze for energy suppliers, which was criticised by business leaders but which sent the message that Labour would be prepared to face down powerful corporations.

“It opened up a debate, and the public had to respond,” said one adviser. “The water cooler debate in the workplace will be ‘that makes sense to me, it’s about fairness’.”

Corbyn’s team hope to play up his unvarnished style and credentials as a political outsider to tap into voters’ frustrations about mainstream politics.