More than two million Syrians are now refugees, with the total going up by a million in six months, the UN says, reports BBC.
At least 700,000 have fled to Lebanon, and more Syrians are now displaced than any other nationality, the UNHCR says.
Around half of those forced to leave are children, UN agencies estimate, with about three-quarters of them under 11.
There is now thought to be one Syrian refugee in Lebanon to roughly every six Lebanese. Jordan and Turkey have taken in the second and third highest numbers respectively.
As well as those who have left the country, a further 4.25 million have been displaced within Syria, the UNHCR says, meaning that more Syrians are now forcibly displaced than is the case with any other country.
The UN says the conflict in Syria has caused the world's worst refugee crisis for 20 years, with numbers not seen since the 1994 genocide in Rwanda.
More than 100,000 people are thought to have died since the uprising against Syrian President Bashar al-Assad began in March 2011.
France and the US are continuing to push for military action over alleged chemical weapons use by Syrian forces.
There are suggestions that President Barack Obama may be planning much wider action than the limited strikes that have been publicly proposed.