There were as late as 1860. American citizen Nathaniel Gordon on the American flagged ship Erie was stopped by the USS Mohican about 50 miles of the coast of Africa. The ship was carrying 897 future slaves. The Africans were taken to Liberia, and Gordon was brought to the United States. He was tried and executed in New York City. Some Americans ships did engage in the slave trade, but as you point out the majority were Portuguese, Dutch or British
As of 1807 slave ships were outlawed, and considers pirates. Thus the ships were not U.S., nor UK flagged ships.