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SPRINGFIELD, Ill. — Republican U.S. Sen. Mark Kirk has apologized for mocking Democratic U.S. Rep. Tammy Duckworth’s immigrant background and her family’s history of military service.

In a Twitter post Friday, Kirk says: “Sincere apologies to an American hero, Tammy Duckworth, and gratitude for her family’s service.”

At a U.S. Senate debate Thursday in Springfield, Duckworth spoke of how her family has “served this nation in uniform going back to the Revolution.”

Kirk responded that he had “forgotten (that her) parents came all of the way from Thailand to serve George Washington.”

Duckworth was born in Thailand to a mother of Chinese descent and a father who first went to Southeast Asia to serve with the U.S. Marines in Vietnam.

The Democratic Senatorial Campaign Committee immediately called Kirk’s comments “wrong and racist.”