"US strikes show needed resolve against barbaric chemical attacks," tweeted Tusk, who represents the 28 EU member states, adding the bloc will work with the US to "end brutality" in Syria.
Jean-Claude Juncker, president of the European Commission, the EU executive, expressed his support for the US decision.
"President Juncker has been unequivocal in his condemnation of the use of chemical weapons," a Commission statement said.
"The repeated use of such weapons must be answered. He understands efforts to deter further attacks," it said.
He also urged diplomatic efforts to end the six-year war in Syria "be redoubled" as only a "political transition" can achieve lasting peace.