Jeremy Corbyn risked a fresh row with Labour MPs over defence policy as he indicated he would never launch a nuclear strike if he was prime minister.
The Labour leader, who said he could "obviously" imagine being in Number 10, stressed he has a mandate from party members for his opposition to renewing the Trident nuclear deterrent.
Mr Corbyn said that "we are not in the era of the Cold War any more" as he defended his stance on Trident but insisted that Labour was not a "divided party".
On BBC Radio 4's Today programme he said: "There are five declared nuclear weapon states in the world. There are three others that have nuclear weapons. That is eight countries out of 192."
Asked would he use nuclear weapons if he was in Downing Street he said: "No. 187 countries don't feel the need to have a nuclear weapon to protect their security, why should those five need it themselves? We are not in the era of the Cold War any more."
He added: "I am opposed to the use of nuclear weapons, I am opposed to the holding of nuclear weapons. I want to see a nuclear-free world. I believe it is possible."
Mr Corbyn also insisted that a political solution rather than military action against Islamic State in Syria was needed because "you don't solve every problem necessarily by going in and bombing".
"There are people in the party who have different views, but what we are all united on is that Isil's behaviour, its actions and its brutality are totally appalling," he told ITV's Good Morning Britain.
"There has to be an end to the fighting. There also has to be a cutting off of the funds and the arms that Isil are using, there has to be a ceasefire amongst the other forces within Syria.
"Maybe some progress has been made on that. You don't solve every problem necessarily by going in and bombing. Basically all wars have to end with a political solution."
Mr Corbyn said it was too early to say whether Labour MPs would be given a free vote in the Commons on military action in Syria, and indicated that he still hopes to reach consensus on the issue within his shadow cabinet.
The Labour leader told the Press Association: "We will have a discussion both here in the conference and within the parliamentary party and, of course, within the shadow cabinet. Maybe we will reach an agreement - I hope we will.
"This is obviously a central and crucial issue. I think what's at the background to this is that many, many people in the Labour Party have dreadful memories of what happened over the war in Iraq in 2003 and the mission creep that's accompanied by what starts off as a simple process - you do some bombing and maybe that doesn't work and then the whole thing gets worse and worse from there.
"I think there's quite a long way to go before we actually reach that conclusion and a decision."
Asked whether he was positioning Labour too far to the left to win over the centre-ground voters the party will need to secure election victory, Mr Corbyn told PA: "Thirty-six percent of the electorate didn't vote in the last election, 53% of young people didn't vote in the last election. Some people were tempted away from us because we were not clear enough on an economic strategy that opposed austerity.
"Instead of cutting, we want to see an expanding economy, an investment-based economy - John McDonnell set that out in his speech on Monday.
"We have a different economic strategy, an economic strategy which actually rebalances our economy, invests in the social needs - particularly in housing, which in turn creates a large number of jobs - invests in infrastructure, which both creates jobs and provides economic opportunities for the growth of manufacturing industry in our economy.
"We need to rebalance things. We have a £100 billion trade deficit at the present time. We have a very low level of investment in our economy. We have a grossly unequal society with the cuts in child tax credits and family tax credits."
Mr Corbyn told the Today programme that Labour faced "huge electoral challenges" next year in Scotland, Wales, London and local elections.