It's claimed thousands of civilians are trapped in the Syrian city of Aleppo following disagreements over an evacuation plan.

Syrian state TV denies the row, but there are reports of people sleeping on freezing streets with very little food.

The government is due to evacuate civilians as part of a truce with rebels, after re-taking the east of the city on Tuesday.

The ceasefire has already collapsed and restarted since then.

Middle East expert Tim Eaton expert explains the situation, using some of the most searched topics on Google.

Who is fighting in Syria?

Broadly speaking, the Syrian government is fighting groups of rebels. But that is a huge simplification.

"Try to think about this as many wars, rather than just one," he says.

"Those different wars interact with each other in different places in different ways. Each of those different groups have conflicts with one another.

"In some places, the regime is also fighting Islamic State. In others, the rebels and Kurdish groups are fighting Islamic State."

The UN says the latest bombing by Syrian-backed forces in east Aleppo was probably a war crime,

How did it all start?

The start of 2011 was known as the "Arab Spring" - a series of anti-government protests across the Middle East.

Syria was a part of it that. People were angry about corrupt officials and a lack of civil liberties.

"As the regime response to it became increasingly brutal and violent, many protestors and others, concluded that they only way they could respond was to arm themselves and overthrow the regime.

"It morphed into a protracted civil war."

Who is President Bashar al-Assad?

He's the man who was never supposed to rule. He was actually training to be an ophthalmologist in London.

Since 2011 we've seen the brutality of the regime and its willingness to adopt violent strategies Tim Eaton Chatham House

But then his older brother died and was recalled back to Syria and groomed to take over when his father died.

"There was actually quite a lot of hope when he succeeded his father in 2000," Tim Eaton explains.

"People felt this might be an opportunity to be more liberal, to open things up.

"Over the first few years, those hopes faded. Since 2011 we've seen the brutality of the regime and its willingness to adopt violent strategies.

"He's a guy who's signed off on some terrible things. "

Why is Russia helping him?

There are a few possible reasons.

"Some people say it's because Russia has a naval base on the Syrian coast. Others say Russia has a keen interest in fighting Islamic extremists."

But really, Eaton thinks it's a global power-play.

"They have put themselves at the top table, seeking to put themselves on a par with the United States."

Who are the rebels?

The rebels is a collective term generally used for the armed groups that oppose Assad. Islamic State are not included in this.

He describes them as a "constellation of entities".

"You can't see the rebels as one group with one point of view, one chain of command and one political message.

"There are many groups. Some are considered as terror groups."

Basically, it just isn't possible to see one side as entirely good while the other is entirely bad.

He talks about "black, white and some grey areas".

"There are rebel groups that the West cannot support, including Jabhat al Nusra which has close links to al-Qaeda.

"And then on the other side, you have Free Syrian Army brigades - many of which the West believes are moderate and are partners.

"In the middle, there are typically Islamist groups which the West doesn't know how to deal with.

"It's not comfortable, but also, it's not willing to put them in the same box as IS.

"The Russians will say those groups are terrorists. But the West doesn't want to say that."

Why is Aleppo so important?

On paper, it's because Aleppo is Syria's second city. So by controlling it, the rebels could present themselves as a credible alternative to the government.

Eaton says their defeat there is the "most significant of the war" and is "obviously very bad for morale."

He argues: "It shows the rebels' weakness rather than the regime's strength.

"It took the regime six months, with massive support from Russia, Iran and other militias to take Aleppo.

"Losing Aleppo doesn't mean the war is over- or even will be over soon."

Could we see another Aleppo?

Since the siege started in Aleppo, we have seen a once bustling city slowly crumbling into ruins.

The news has shown us grim images of extreme human suffering.

Tim Eaton doesn't think this is a one-off.

"It's taken six months for the regime to take Aleppo in a brutal bombardment.

"The rebels hold territory elsewhere. So the sad truth is, we're going to see the exact same tactics elsewhere.

"All of the horror is going to happen again."

What is the true scale of suffering in Syria?

"The truth is, it's absolutely terrible," he says.

"It's hard to give an accurate sense of scale, but most accept that over 500,000 have been killed."

He says that more than a "mind-blowing" 11 million people have either fled their homes or the country.

That is half of the pre-war population.

Why have countries like the UK stood by and let this happen?

Like everything else in Syria, there is no easy answer to this.

Eaton says that at first, there was hope that peace talks would be successful.

In 2013, hundreds of people were killed in a suspected chemical weapons attack.

Countries like America and France said only the government could have been responsible. They threatened military strikes.

In the UK, MPs voted against bombing Syria. This was a key moment.

"That kind of set the tone. Assad was able to conclude that nobody would throw him out.

"That meant he could be as brutal as he liked, as long as he didn't use chemical weapons."

How can people like me help?

Tim Eaton says one option is to donate to charities that try to work in Syria, even though it's getting harder for them to operate in a warzone.

"Really, I think people can make sure the issue remains on the agenda for our politicians.

Read about how to help other people this Christmas here

"The events in Aleppo can't just be seen as something that just happens.

"Frankly, we're not seeing enough responsibility being taken on that front. We need to hold our politicians accountable."

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