shortly after being approached by Israeli vessels, a spokeswoman said.
"The Estelle is now under attack - I have just had a message from them by phone," Victoria Strand, a Stockholm-based spokeswoman for the Ship to Gaza Sweden campaign told the AFP news agency on Saturday.
According to Dror Feiler, another spokesperson, the Estelle, whose passengers include five parliamentarians and a former Canadian politician, was attacked at around 08:15 GMT.
"Five or six military vessels surrounded the Estelle. Soldiers wearing masks are now trying to board the ship. The attack took place on international water: N31 26 E33 45," Feiler said.
The Israeli military confirmed that navy vessels were in the vicinity of the Estelle but denied the ship had been attacked or boarded.
"The Israeli navy has attempted to make initial contact with them, but they have not answered and nothing further has happened at this stage," a spokeswoman said.
Asked whether any attempt had been made to stop or board the Estelle, she said: "Not at the moment."
The Estelle, the latest ship to try to break Israel's blockade on Gaza as part of the 'Freedom Flotilla' movement, set sail from Naples in southern Italy.
Luigi de Magistris, the mayor of Naples, met with members of the crew during a visit to the Estelle in the city's port.
The Estelle, whose voyage is being organised by an international pro-Palestinian coalition, is carrying humanitarian goods to the Gaza Strip.
There are 17 activists from around the world on board, including passengers and crew from Canada, Israel, Norway, Sweden and the US.