Video courtesy of Infolive.tv
The ship, Al-Marwa, sailed instead to a port in neighboring Egypt. It left Libya last Monday, carrying 3,000 tons of food, medicine and other aid, Palestinian and Libyan officials said.
Imposed to generate political pressure on Islamist Hamas, the blockade has been stepped up in recent weeks amid a surge in border clashes between Israeli forces and Palestinian militants.
Israel, which has in the past allowed ships carrying humanitarian goods to dock in Gaza to avoid a public confrontation, said Al-Marwa had turned back without incident and that it did not know where the boat was headed.
They understood that the navy was there and decided to turn around," said Andy David, an Israeli Foreign Ministry spokesman. "We have a very clear policy (on the blockade) which is constantly publicized."
Palestinian officials said the vessel had left Gaza's shores and its crew was unharmed.
The Anti-Siege Committee, a Palestinian group that lobbies with Hamas backing against the embargo, accused Israel of having also blocked Al-Marwa's communications broadcasts to Gaza. The ship was now docked in Egypt's El-Arish port, the group said.
"We urge all sides to pursue this intifada (uprising) of ships," said Anti-Siege Committee spokesman Ala al-Batta.
Roni Sofer contributed to this report