(Releads on boat returning to Cyprus)

NICOSIA, Jan 12 (Reuters) - Activists sailing to Gaza with medical supplies in defiance of an Israeli naval blockade had to turn back to Cyprus on Monday when their vessel developed engine trouble, organisers said.

Members of the U.S. based Free Gaza Movement had planned to take doctors, aid workers and European parliamentarians on a small ferry to Gaza, where Israel has mounted an offensive to rout Hamas fighters.

The group said the Greek-flagged boat had suffered problems with a generator about three hours off the Cypriot coast.

"The boat is on its way back. The problem is going to be repaired and we are going to go back (to Gaza) as soon as the boat is in shape to go," a representative of the Free Gaza Movement told Reuters.

Earlier, activists said they were determined to get into the Palestinian enclave.

"We cannot wait until Israel decides to stop the killing and then open up Gaza for the international community and aid workers to pick up the pieces," Huwaida Harraf, one of the organisers, said.

"We insist on the right to enter, we are not going to let the violence Israel may use against us stop us," she said.

Israel launched its offensive on Gaza with the declared aim of ending cross-border rocket attacks from the territory.

Medical officials said the Palestinian death had risen past 900 and included at least 380 civilians. Israel says 13 Israelis have died -- three civilians hit by rockets and 10 soldiers.

The group of 34 had acquired the small ferry after another vessel was involved in a collision on Dec. 30 with an Israeli boat when it was taking aid to Gaza from Cyprus.

The trip, with parliamentarians from Greece, Spain and Belgium on board, was expected to take about 25 hours. Free Gaza has sent regular shuttles of aid to Gaza from Cyprus since August.

(Editing by Michael Roddy)

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