Israeli forces have intercepted a boat carrying pro-Palestinian activists seeking to breach the naval blockade on Gaza.

A military spokeswoman said no one was hurt when marines boarded the Estelle, which was rerouted to Israel's southern Mediterranean port of Ashdod.

A campaigner said the boat had "come under attack" after being pursued by Israeli naval vessels.

Activists said they lost contact with the Swedish-owned, Finnish-flagged boat early on Saturday.

"The Estelle is now under attack. I have just had a message from them by phone. Some time ago, they said that they had military ships following them," Victoria Strand, a Stockholm-based spokeswoman for the Ship to Gaza Sweden campaign, told Agence France Presse (AFP).

She added that it was unclear what the activists onboard meant when they said they had been attacked.

The Estelle is carrying 30 activists from Europe, Canada and Israel, humanitarian cargo such as cement, and goodwill items such as children's books.

Israeli army radio reported that its naval forces had ordered the Estelle to halt its course.

The Estelle is the latest in a series of vessels manned by activist that have tried to challenged Israel's blockade on Gaza, imposed after Hamas seized power in 2007.

It left the Italian port of Naples on 7 October with about 20 people from eight countries on board.

Israel maintains a tight naval blockade of Gaza, which it maintains is necessary to prevent arms smuggling to Hamas and other Palestinian militants.

Palestinians describe the blockade as a collective punishment on Gaza's 1.6 million residents, and supporters abroad have mounted several attempts to break it by sea.

Nine Turkish activists were killed in May 2010 when Israeli naval commandos boarded their flotilla of six ships.

An inquiry into that incident commissioned by UN secretary-general, Ban Ki-moon, found that Israel's Gaza blockade was legal but criticised the navy for using excessive force.