Aid organisations in Greece and the Balkans have been warning for months there was trouble on the way.

Now it has arrived, in the shape of a mass of cold air from Siberia that brought sub-zero temperatures and driving snow and sleet to ill-prepared refugee camps in south-east Europe.

Ariel Ricker runs Advocates Abroad, a team of 200 asylum law experts.

She spends much of her time these days on the Greek island of Lesbos in the now-infamous Moria camp – a sprawling tentland that's home to thousands of migrants.

"No-one was expecting a snow storm of this intensity… and now we're dealing with freezing rain so it's just been one disaster after another," she says.

Usually they would be flat out with the plodding asylum process: helping refugees prepare for their interviews, negotiating administrative hurdles.

But the Moria residents' needs have become more immediate.

"Right now my biggest concern and what I spend a lot of my time on isn't legal, per se," Ms Ricker says. "It's making sure people are safe."

At the start of winter a gas canister in a tent (many refugees use them for heating or to cook food) exploded, killing two people and injuring more. Ms Ricker went through the camp, tent by tent, to check gas canisters' integrity.

On Friday she spent hours trying to ensure that local hoteliers would honour a promise to put up a family of refugees. Last week she got in a car at 2am when she heard there was a room with heating she could move a client to.

There is pneumonia and bronchitis. They're seeing a lot of hypothermia. But there's a psychological aspect to the conditions as well, she says. Most Moria residents have been there for months. The sub-zero temperatures, snow and rain "deaden their spirit", she says.

She has noticed a rise in self-harm, too. In a way it's worse than the riots, which she sees at least as "an affirmation of life". But migrants who attempt suicide sometimes get fast-tracked to interviews. And when word of that spreads there is a spate of "cutting, drinking, poison, drugs".

"It's really upsetting actually," she says.

As a lawyer she has been shocked to find that the anarchists run a good kitchen, while some police beat children: her group recently took a complaint to the local ombudsman which ended with a policeman facing criminal charges for beating a minor.

But, she says, "I can also understand that (police) are under a great deal of stress. They are the only line of defence – in their minds – between this incredibly chaotic situation and their country".

In response to the latest crisis Greece dispatched a navy ship to the islands, but it can only take hundreds on board where thousands are in need of help.

Ms Ricker says it is shocking how little help the EU is providing Greece. A group of Belgian officials were pulled off the islands after the last riot, and they have been unable to find people to fill much-needed caseworker and translator positions.

Meanwhile, border protection organisation Frontexthas hired 600 new employees.

"You would think that they would want to put the emphasis on those who can process (asylum) claims and get these people off these islands rather than just security," Ms Ricker says.

Meanwhile, on the mainland, Belgrade is about to see a week of sub-zero temperatures.

Ninja Taprogge, of CARE International, says there are more than 7000 migrants in Serbia of whom around 2000 are unregistered and living in Belgrade.

Because of their status they cannot move to the well-equipped, heated refugee camps, where there are warm meals and classes and recreational opportunities for the children..

Instead they camp in abandoned warehouses in the city centre in extremely difficult conditions, Ms Taprogge says.

"I have seen people walking around in flip-flops (thongs) and bare feet, no warm clothes, they don't have proper winter jackets," she says. "They don't have access to water. They don't have access to sanitation facilities. It's just inhuman, inhumane conditions."

The only thing keeping them going is "strong willpower", she says.

But even those who survive the winter face a bleak future.

Single male refugees who have made it to Serbia may have to wait an average two years before being able to leave the country, due to the glacier-like asylum bureaucracy.

"The European Union really needs to step up," Ms Taprogge says. "It's important that the EU comes together and provides safe and legal options, to give people the chance to seek asylum who are in fear of their life – because all of them, or many of them fled war.

"The only option for refugees is now to pay money to smugglers, and risk their lives on dangerous paths."