At the final summit of one of the most turbulent years in the EU's six-decade history, the bloc's 28 leaders also discussed preparations for Britain to become the first nation to leave the bloc.
EU President Donald Tusk said they had agreed to push Syria, Russia and Iran using "all diplomatic means necessary" to open humanitarian corridors under international supervision for civilians in Aleppo.
But after a meeting in which the leaders heard an emotional plea from the mayor of besieged eastern Aleppo, Tusk acknowledged that the bloc was largely powerless to intervene.
German Chancellor Angela Merkel said the discussion on Syria was "very depressing" and that it showed the EU could not act politically in the way it wanted.
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Brita Hagi Hassan, the local official in charge of eastern Aleppo, had earlier warned the leaders that "the 50,000 citizens of eastern Aleppo are only waiting for their deaths after the failure of the international community".
The summit also discussed the migration crisis triggered largely by the Syrian conflict, with Tusk saying the EU planned a summit with Turkey in January to discuss a troubled deal with Ankara aimed at reducing the flow of refugees.
But it flexed the only muscle it does have, renewing for six months damaging economic sanctions imposed against Russia in 2014 over the conflict in eastern Ukraine.
Tusk admitted that some EU countries had wanted a 12-month rollover but that it was never really an option.
Meanwhile, the EU leaders reached a deal with the Netherlands to stop the country torpedoing the bloc's crucial association agreement with Ukraine, which Dutch voters rejected in a referendum in April.
The EU agreed the cooperation pact and an associated free trade agreement with Ukraine in 2014 after pro-EU protestors ousted Russian-leaning president Viktor Yanukovych, who fled to Moscow.
But the EU leaders gave the Dutch guarantees that the pact was not a precursor to Ukraine's membership of the union or to military aid for Kiev.