A new bleak milestone has been reached in Syria, with the number of refugees leaving the country now at more than 4 million.

They have been driven from their homes because of conflict in the country, which is now entering into its fifth year.

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The United Nations (UN) has described it as the world's largest refugee crisis in nearly a quarter of a century, with children making up nearly half the number of refugees.

"You have children who have been born into this war, this is the fourth or fifth year, they have not seen anything but violence. They are very traumatised," Save The Children spokeswoman Nadine Haddad said.

In 2012, Ms Haddad was based in Jordan helping refugees who had fled from neighbouring Syria.

Earlier this year she returned to the Middle East to find a crisis that has only become worse.

"We have Syrians who spend 90 per cent of their earnings for rent and then they have very little left to provide for the needs of their children," she said.

"That forces families to resort to negative coping mechanisms, like early marriages.

"A lot of young Syrian girls are forced to marry at a young age because to a father, as a father, he says 'one less mouth to feed is a way I can protect my family, is the only way I can protect my family'.

"You have a lot of children who choose to go to work or are forced to go to work and a lot of them end up working in exploitative environments.

"Then of course, the worst outcome of all would be the possible radicalisation of those young people due to a lack of opportunities."

While most Syrians who have made it out of the country live as refugees in neighbouring states, the UN has said many more people remain displaced within Syria.

"With the most recent arrivals of Syrians into Turkey, we've reached a very grim milestone," UN refuge agency senior advisor Jana Mason said.

"Which is that we've now reached, and in fact surpassed, a number that we've been anticipating for some time, which is 4 million refugees, Syrian refugees in neighbouring countries.

"I should mention that this is in addition to a larger number, probably at least 7.5 million Syrians who are internally displaced within the borders of their own country, within Syria."

UN lacks funds to help refugees

The UN said it did not have the funds it needed to help all the Syrian refugees.

It appealed for $5.5 billion this year and so far has only received a quarter of that amount.

"You know, it's hard to criticise donors because they've been very generous," Ms Mason said.

"I think the needs just far exceed clearly the funding that's been pledged and also maybe just the ability of donors, many of whom are facing difficult economic times, to keep funding this crisis. Certainly we need more."

The UN said not only is more funding urgently needed, but the international community needs to help resettle more refugees.

"Obviously most of the brunt of this, the burden is being borne by Syria's neighbours, who themselves are starting to buckle under the strain," Ms Mason said.

"One way we can help those countries, in addition to giving them financial aid, is to relieve some of the numbers by Europe, the United States, other countries resettling Syrians, offering asylum to Syrians.

"Because what we're seeing is that, as Syrians in the neighbouring countries become more desperate, you've seen them make that risky move through boats on the Mediterranean, going into Turkey, into Greece or elsewhere in Europe and they're really risking their lives to flee to safety.

"So clearly we need the international community to do more in terms of shouldering the responsibility."

Permanent solution to end fleeing needed: UN

But ultimately the UN said there needed to be a permanent solution which addressed why people were fleeing Syria.

There are young Syrians who have been born into the conflict, which is entering its fifth year. ( UNHCR: Laban-Mattei )

"Even though it's a humanitarian crisis, the solution won't be humanitarian," Ms Mason said.

"The solution will be when the international community, together, helps Syria arrive at a negotiated political settlement to the conflict."

More than 200,000 people have been killed in Syria's war since 2011, sparked by a violent crackdown on anti-government protestors.

US president Barack Obama is among international leaders who say Syrian president Bashar Al Assad must step down for the war to end.

The US wants the UN Security Council to investigate who is responsible for chlorine gas attacks in Syria.

Mr Assad is accused of inflicting harm on civilians through the use of chemical weapons but denies his forces are to blame.