NAIROBI, April 11 (Reuters) - Pirates seized a U.S.-owned and Italian-flagged tugboat with 16 crew on Saturday in the latest hijacking in the busy Gulf of Aden waterway, a regional maritime group said. Andrew Mwangura, of the Mombasa-based East African Seafarers' Assistance Programme, said the crew were believed to be unharmed on the tugboat, which he added was operated from the United Arab Emirates.