General Thomas Waldhauser, Commander of US Africa Command, said there had been half a dozen attacks in the last month.
Some of the vessels seized were carrying food and oil, he added.
After peaking in 2011, the number of attacks had dropped to zero, mainly as a result of naval patrols and improved security measures by shipping companies.
General Waldhauser was speaking during a visit by the US Defence Secretary, Jim Mattis, to the American military base in Djibouti.
Mr Mattis said the rise in piracy was being watched but he wasn't expecting a US military response.