The Syrian conflict has caused the world's largest refugee crisis as it has forced 40 percent of the population from their homes, the UN refugee agency said Friday in Geneva.

More than 9 million Syrians have been displaced since the conflict started three years ago, the Office of the UN High Commissioner for Refugees (UNHCR) said.

More than 6.5 million have sought refuge within Syria, while 2.6 million have fled to countries in the region.

"It is unconscionable that a humanitarian catastrophe of this scale is unfolding before our eyes with no meaningful progress to stop the bloodshed," said High Commissioner Antonio Guterres.

He noted that the overwhelming majority of the refugees have been taken up not by wealthy countries, but by Egypt, Iraq, Jordan, Lebanon and Turkey, overstretching economies and infrastructures.

If other countries took in as many Syrians per capita as Lebanon, there would be 19 million Syrian refugees in Germany and 73 million in the United States, the UNHCR calculated.

Only a small fraction of Syrians have sought protection in Europe, excluding Turkey, often taking dangerous trips across the Mediterranean and risking being pushed back to other countries on their way there.

More than 146,000 people have been killed in Syria during the conflict, the pro-opposition Syrian Observatory for Human Rights said Thursday.