The Israeli navy ordered a small ferry carrying medical supplies and foreign peace activists trying to break a blockade of Gaza to turn back, a spokesman for the group said on Tuesday.
The Israeli military would not confirm or deny the report. A statement by the voyage's organizers, the Free Gaza Movement, said "the vessel, the Spirit of Humanity, left the Cypriot port of Larnaca on Monday bound for Gaza with 3 tons of medical supplies."
The 20 passengers include former U.S. Representative Cynthia McKinney, Nobel Peace Prize winner laureate Mairead Corrigan Maguire and other activists from Britain, Ireland, Bahrain and Jamaica.
"There is a patrol boat around us and we were told that if we did not turn back they would open fire," the movement's Web site quoted Derek Graham, an Irish activist, as saying by satellite telephone.
The Free Gaza Movement has organized five boat trips to Gaza since August 2008, defying a blockade imposed by Israel when the militant group Hamas seized control of the territory from its Palestinian rivals in June 2007.
Two other voyages were stopped by Israeli warships during Israel's three-week war in the territory in December and January. Nobody on board was harmed.