The United Nations on Wednesday predicted the number of Syrian refugees in neighboring countries will double to 1.1 million by June next year if the country's war is not ended.

There are now more than 540,000 registered refugees in Iraq, Jordan, Lebanon, Turkey and Egypt, according to the latest UN Office for the Coordination of Humanitarian Affairs (OCHA) report on Syria.

That figure has increased by more than 140,000 in six weeks. "Syrians continue to cross borders to seek safety," OCHA said.

The UN High Commissioner for Refugees "estimates that if fighting in Syria continues the refugee figure could reach 1.1 million by June 2013," the report added.

Aid groups say hundreds of thousands of Syrians have fled the 21-month old conflict without registering with UN agencies.

The United Nations has appealed for $1 billion to fund refugee operations until June.