The captain of a Norwegian boat carrying pro-Palestinian activists en route to Gaza has accused Israeli authorities of violating the law by boarding the vessel in international waters and using violent force against its crew.

Norway has asked Israel to explain the circumstances surrounding the arrests and "the allegations of excessive use of force," a Norwegian government spokesman told AFP on Thursday.

"We were arrested in international waters and we were closer to Egypt than Israel," the boat's captain Herman Reksten said early Thursday when he returned to Norway after being held for three days in an Israeli prison.

"Israel has broken all the rules, it's horrific that they board a Norwegian ship in international waters and force it to moor in Israel," he told Norwegian public broadcaster NRK.

The Israeli army on Sunday announced the seizure of a boat off the Gaza Strip with activists who were denouncing the land and sea blockade imposed by the Jewish state on the Palestinian enclave for more than a decade.

The Norwegian-flagged Karstein ship had 22 people on board.

Several have been released since, but seven people -- two Norwegians, two Swedes, one French, one Spaniard and one Canadian -- were still detained on Thursday according to the rights organisation Just Future for Palestine.

As he arrived at Oslo International Airport, Karstein captain Herman Reksten also accused Israeli soldiers of shocking the activists with tasers.

"I still have a headache from being hit in prison," he added.

Contacted by AFP, the Israeli embassy in Oslo was not immediately available for comment.

"We've asked the Israeli authorities to clarify the course of events and on what basis they think they are entitled to intervene on the ship," Norwegian foreign ministry spokesman Frode Andersen told AFP in an e-mail.

"We will also raise the allegations of excessive use of force," he added.

Oslo says it has not yet received a response.