The civil war in Syria has created an unprecedented humanitarian crisis, according to the United Nations High Commissioner for Refugees (UNHCR), as it has forced more than three million people to flee the country – an increase of one million in the past year alone.

UN High Commissioner for Refugees António Guterres called the crisis “the biggest humanitarian emergency of our era” with almost half of all Syrians forced to abandon their homes since the conflict began in March 2011.

“The Syrian crisis has become the biggest humanitarian emergency of our era, yet the world is failing to meet the needs of refugees and the countries hosting them,” he said.

“The response to the Syrian crisis has been generous, but the bitter truth is that it falls far short of what’s needed,” Mr Guterres said.

The agency said its work to help the Syrian refugees now marked the largest operation in its 64-year-history.

A total of 6.5 million Syrians have been displaced from their homes, representing about 30 per cent of Syria’s pre-war population of 23 million.

More than half of all those who have been uprooted are children, the UN says, describing “cities where populations are surrounded, people are going hungry and civilians are being targeted or indiscriminately killed”.

Most of the Syrian refugees have found their way to neighbouring countries, with Lebanon hosting 1.14 million, Jordan 608,000 and Turkey 815,000.

The influx is placing an overwhelming burden on the host countries, the UNHCR stressed, adding that nearly 40 per cent of the refugees were living in sub-standard conditions.

The UNHCR said families were arriving in neighbouring countries “in a shocking state, exhausted, scared and with their savings depleted.”

The agency also voiced deep concern for several hundred Syrians trapped inside the remote al Obaidi refugee camp in Iraq after UN agencies and other groups were forced to abandon their offices and warehouses as the region became overrun by the Islamic State group.

Many newly arriving refugees say they only left Syria as a last resort. A growing number, including more than half of those coming to Lebanon, have moved at least once before fleeing, and one in ten have moved more than three times.

The increasingly fragmented conflict raging in Syria has claimed more than 191,000 lives since erupting in March 2011.

In the past year alone, 1.7 million refugees received food aid, 350,000 children were enrolled in school, and shelter in camps was provided for more than 400,000 refugees.

Since the beginning of the crisis in 2011, UNHCR has registered refugees faster than at any time in its history.

Donors have contributed more than US$4.1 billion to successive regional response plans since 2012. But more than US$2 billion more is needed by the end of this year alone to meet the urgent needs of refugees.

This week Australia committed RAAF aircraft to deliver weapons to Kurdish Peshmerga fighters battling the ISIL Jihadists whose campaign to found a fundamentalist Islamist Caliphate in the region has greatly intensified the conflict in Syria.

Prime Minister Tony Abbott has also refused to rule out further involvement including a direct military role.