Jeremy Corbyn has refused to say whether he would deploy British troops to defend a Nato ally invaded by Russia.
The Labour leader said he was "not in favour of anybody invading anywhere" and would do everything he can to de-escalate tensions with Russia now.
It comes amid heightened concerns over Russian aggression and the future of Nato after President Donald Trump complained the United States has "subsidised the armies of other countries" and "defended other nations' borders while refusing to defend our own".
Baltic nations fear a Russian incursion as Vladimir Putin masses troops on their borders.
Britain has responded by sending 800 soldiers backed by tanks and drones to join a Nato show of strength in Estonia.
Under the Nato mutual defence pact, an invasion of a Baltic member of the alliance would be seen as an attack on all members.
Asked if he would deploy UK troops to uphold the pact in the event of a Russian incursion, Mr Corbyn told Sky News' Sophy Ridge on Sunday: "I would want us to try and de-escalate the tensions as of now.
"And there are some signs that could happen.
"That means building up a positive relationship with Russia - Nato-Russia talks are important, EU-Russia talks are important.
"And on that, include issues of human rights and justice in Russia, include removal of troops away from the border."
Asked again if he would commit troops, Mr Corbyn said: "It's a hypothetical question.
"I'm not in favour of anybody invading anywhere else and I would do everything I could to bring about a situation where there are reasonable, productive relationships between all countries so we don't end up in a situation where lives are put at risk and people's liberties are at risk as well."