Barroso spoke by phone with Ukrainian President Viktor Yanukovych to express frustration with escalating violence in Ukraine.
The commission said in a statement if the situation "is not stabilized, the European Union would assess possible consequences for bilateral relations."
A November decision by the Yanukovych administration to suspend efforts to sign free trade and association agreements with the EU sparked widespread protests in Kiev. Protests turned deadly during the weekend after the president backed legislation restricting demonstrations.
A Thursday statement from the ruling Party of Regions, published by the National News Agency of Ukraine, said supporters of the president pledged to protect the capital from "right-wing extremists."
Yanukovych was urged by the EC president to start dialogue with members of the opposition "immediately."
Barroso said EU Neighborhood Policy Commissioner Stefan Fule was headed Friday to Kiev and Catherine Ashton, the EU's top foreign policy official, may head to the country next week.
Yanukovych said the November decision was made to protect economic ties with Russia.
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