Up to 20,000 Syrians may be stranded on the border, officials say, trying to get into Turkey which already hosts more than two million refugees from the bloody conflict.
"The Geneva convention is still valid which states that you have to take in refugees," EU Enlargement and Regional Policy Commissioner Johannes Hahn said as he went into talks on the migrant crisis with EU foreign ministers and their counterparts from countries seeking EU membership, including Turkey.
Luxembourg Foreign Minister Jean Asselborn said "everyone has seen the pictures from Aleppo, the tens of thousands of people who are fleeing, fleeing for their lives."
"We face the very real prospect that there will be another huge influx of refugees (into Europe)... That is the result of the indiscriminate bombing around Aleppo," Asselborn said as he went into the meeting.
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More than a million migrants landed in the 28-nation European Union last year, most of them crossing into Greece from Turkey, and then making their way through the Balkans to Germany and other northern member states.
Such numbers have put huge strains on the bloc and the Schengen passport-free zone, with several countries - among them Germany, Austria, Hungary, Sweden - re-introducing border controls while Brussels struggles to find a comprehensive solution.
In November, the EU thrashed out a deal with Turkey, offering 3.0 billion euros to help care for the refugees on its soil and speeding up long-stalled accession talks in return for Ankara's help in curbing the migrant flows.