Jeremy Corbyn has said no more British troops should be sent to the Baltic states as he called for increased dialogue with Russia.
The Labour leader also refused to say if he agrees with the Nato principle of collective defence - which means an attack on one member is deemed an attack on all.
Instead, Mr Corbyn called for greater efforts to demilitarise the eastern border with Russia.
Asked if he considers it a "mistake" to have sent British troops to the region, the Labour leader told ITV's Peston On Sunday: "I'm saying that has happened, I don't think it should be increased.
"I think we should be looking to try to reduce tensions within the border area.
"I think that surely is important because surely we need to have dialogue, dialogue in a meaningful way, reopen the dialogue with Russia as quickly as we can.
"It will also help, of course, in bringing about a better dialogue in the dreadful situation in Syria and bring about a real ceasefire."
Mr Corbyn's spokesman has previously suggested Britain's deployment of troops and jets to Estonia was "escalating tensions" with Russia.
The Labour leader also dodged questions over whether he is "equivocal" in his support for Nato and instead stressed the need to scale down military tensions.
He told the show: "It has expanded quite a lot towards the east, Russian militarism has grown quite a lot.
"My point has always been there has to be an improved and deepened dialogue between Nato and Russia in order to demilitarise those borders in order to reduce tensions in the area.
"Both sides sending troops to the border isn't doing anything about that long-term situation."