A top Russian official has warned Ukraine against signing a landmark trade and cooperation agreement with the European Union, saying Moscow would retaliate with trade restrictions that could push this ex-Soviet republic toward default.

Speaking at a conference in the Black Sea city of Yalta on Saturday, Russian presidential adviser Sergei Glazyev dismissed the benefits of a planned free-trade deal between the EU and Ukraine as "mythology." He warned that tariffs and trade checks that Russia would impose after the deal could cost Ukraine billions of dollars and result in a default.

"One has to be ready to pay for that," Glazyev said.

Polish Foreign Minister Radoslaw Sikorski, however, urged Ukraine to resist the Kremlin pressure and sign the agreement in November, citing Poland's success in joining the bloc.