The United Nation's refugee agency warns as many as 700,000 people could have fled Syria by the end of the year.
Some 294,000 refugees have already left Syria, and the UNHCR is appealing for $US490 million to deal with the crisis.
Most of the refugees are housed in camps in Turkey, Lebanon and Jordan, the BBC reports.
Meanwhile, rebels in Aleppo say they have launched a "decisive" offensive on the city, where they have been battling government forces for weeks.
A British-based activist group says Wednesday was the bloodiest day of the 18-month conflict so far, with more than 305 people being killed across the country.
Rami Abdel Rahman, from the Syrian Observatory for Human Rights, said the figure included only those whose names had been documented.
"If we count the unidentified bodies, the figure will be much higher," he said.
The observatory said 199 of Wednesday's dead were civilians.
It is estimated that more than 30,000 people have been killed overall in violence since the outbreak of the revolt against President Bashar al-Assad in March 2011.