The number of Syrians who have fled their conflict-riven homeland has hit one million since fighting broke out two years ago, the UN's refugee agency said Wednesday.

"With a million people in flight, millions more displaced internally, and thousands of people continuing to cross the border every day, Syria is spiralling towards full-scale disaster," UN High Commissioner for Refugees Antonio Guterres said in a statement.

"We are doing everything we can to help, but the international humanitarian response capacity is dangerously stretched. This tragedy has to be stopped."

The UNHCR said the level of one million comprised both registered refugees and those awaiting registration, and that the count was based on fresh data received from its offices in the Middle East.

The agency previously had estimated that numbers would reach 1.1 million by June but said Wednesday that it would adjust that figure.