Iraq -- 168,000
Government ministers from those four countries will meet with the UNHCR in Geneva Wednesday in an effort to generate greater international support for dealing with the refugee exodus.
Wissam Tarif, executive director of the human rights group Insan, which is building a refugee camp in Lebanon, encouraged residents in neighboring countries to open their homes to refugees as prospects for housing them become thinner.
"We will be able to host up to 1,200 families. That's around 10,000 people, but that's nothing compared to the influx that's expected. Lots of people will stay in the streets," he said.
More than half of the 2 million Syrian refugees are children age 17 or younger, the United Nations said.
The British Red Cross suggested that the U.N. figure for the total number of refugees may well be too conservative.
"To have reached this landmark figure of 2 million registered refugees is shocking, but the true figure is likely to be higher," said Pete Garratt, a disaster manager for the British group. "We know there are people who will not have registered for support, for many reasons. They may be afraid of any form of authority or of registering their status."
The organization said that in Jordan, 70% of refugees live in urban areas rather than in camps, which makes it harder for aid agencies to find and help them.
"Our colleagues from the Jordanian Red Crescent report finding families who have not registered, or are worried about coming forward to ask for support," Garratt said. "They are still living with the psychological effects of having been in a conflict zone, and that makes people wary."