Labour leadership contender Andy Burnham has accused his rival Jeremy Corbyn of "making excuses" for Russia's President Vladimir Putin over the country's actions in Ukraine.
The comment was in response to Corbyn's criticism of NATO and its "excessive" and "obessive" expansion and his claim that the Russian military has pressured its government over the Ukraine crisis.
Corbyn branded what Burnham said as "ridiculous."
The clash was during a live broadcast of a Labour leadership debate, hosted by Channel 4 News.
The four contenders - Andy Burnham, Jeremy Corbyn, Yvette Cooper and Liz Kendall - disagreed on a number of issues from immigration to military intervention in Syria.
Here are some of the main topics discussed:
Yvette Cooper was the only candidate to give actual figures for the number of refugees she thinks the UK should take, repeating her calls from a speech she made this morning that the country should take 10, 000.
Her rival Andy Burnham said that he thought if Britain did take in more refugees the country would get a better hearing from other EU countries on the issue of EU migration.
Cooper made it clear that we need to separate the issues of asylum and immigration as they are different things.
Islamic State in Syria
Andy Burnham said he would not rule out military action against Islamic State in Syria. But the Labour MP emphasised that following the aftermath of the 2003 invasion of Iraq, legality of intervention in Syria would be "a major question".
Cooper and Kendall did not rule it out either, with Cooper saying she would have to look seriously at it. Kendall warned that air strikes would have to be part of a wider strategy in the region.
Jeremy Corbyn said that it was very hard to see any circumstances in which he would support military action against ISIS in Syria - and that it could even make the situation worse.
Tony Blair and the Chilcot Inquiry
None of the candidates ruled out supporting the prosecution of Tony Blair over the invasion of Iraq in 2003 if there were charges after the Chilcot Inquiry.
Burnham did not hold the view that the former prime minister knowingly misled people and said that at the time there was widespread support - including from the Kurds - for intervention.
Kendall argued that Labour would need to reduce welfare bill, with Burnham saying that there was something very wrong with the way the benefits system worked.
He said that the process of benefits sanctions can be punitive and was "driving people to despair." The Islington North MP stated that Labour should oppose the Conservatives' benefit reform bill, including the decision to cut the household benefits cap.
We've got to oppose this bill in its totality for what it's trying to do - it's trying to hurt the most vulnerable and weakest in our society.
Cooper criticised the government's current strategy of difficult targets and penalising people that show up late to interviews.