John Kerry opened up about his time as US secretary of state in an in-depth interview published on 18 December, and revealed that a solution to limit Russia's military activities in Syria was blocked because of "divisions" within the US political ranks.

In his final weeks in the role, Kerry continues to focus on the Syrian civil war and the civilians of Aleppo, who are being killed and displaced by the Syrian military as it moves to reclaim control of the city.

"We're just trying to find a way to stop the violence," Kerry told The Boston Globe's Matt Viser. "And it's very difficult."

Kerry, who worked on the Iran nuclear deal and helped persuade nearly 200 countries to sign up to a climate agreement, revealed that he had negotiated a deal with Russia to share joint military operations but it fell apart.

"Unfortunately, we had divisions within our own ranks that made the implementation of that extremely hard to accomplish," he said. "But I believe in it, I think it could have worked."

Kerry noted that the plan cannot be implemented now "because of what's happened to Aleppo. But the fact is we had an agreement in which Russia gave us a veto over their flights and over what they were doing in the area, had we set up a joint cooperative effort.

"Now, we had people in our government who were bitterly opposed to doing that," he added. "I regret that. I think that was a mistake. I think you'd have a different situation there conceivably now if we'd been able to do that."

Aleppo has been on the receiving end of increased military action and bombings as the Syrian regime fights to reclaim the last rebel stronghold in the east. The Globe noted there have been reports of women and children being massacred as they attempt to flee the city.

Kerry has tried to establish a humanitarian corridor and UN monitoring system to help residents fleeing Aleppo. The 2004 Democratic presidential nominee was open about his frustrations against Russia and Syrian President Bashar al Assad.

"The Russians and Assad are basically throwing everybody into the same pot and acting way outside any decent standard of warfare ... and challenging all of the norms of expected humanitarian behaviour," Kerry said.

He said a ceasefire and Geneva talks between the warring sides is "the only ultimate solution to the war" but noted that being "able to patch the country back together" is a major issue. "I'm trying to line it up so the opposition will understand and embrace the idea of having those talks, even at this difficult moment. And it's hard," he told The Globe.

Kerry's replacement will be ExxonMobil CEO Rex Tillerson, who will be nominated by President-elect Donald Trump. Kerry told The Globe that after 34 years in public office, he plans to return to Boston and join the private sector.