Reverberations from the Ukraine war widened on Wednesday, jolting energy markets and spilling across borders, as Russia responded to the West’s escalating arms shipments and economic penalties by halting gas supplies to two European nations and threatening further unspecified retaliation.

The European Union’s top official described as "blackmail” the announcement that Russia was suspending shipments of natural gas to Poland and Bulgaria. Though the immediate effect was likely to be limited, the cutoff was the Kremlin’s toughest retaliation yet against a U.S.-led alliance that President Vladimir Putin has accused of waging a proxy war aimed at weakening Russia.

Even as news of a U.S.-Russia prisoner exchange offered a glimmer of hope for diplomatic engagement, Putin warned that he would order more "counterstrikes” against any adversaries that "create threats of a strategic nature unacceptable to Russia.”