The aid ship trying to break the Gaza blockade has ignored repeated demands from Israeli warships to divert to the Israeli port of Ashdod.

The Israeli military says it has made radio contact with the 1200-tonne Rachel Corrie, urging it to change course for Ashdod and not Gaza, because of the ongoing blockade.

"A short time ago we have contacted the boat and we've asked them politely to change their course to Ashdod port," said Israeli military spokeswoman Lieutenant-Colonel Avital Liebovich.

"If they will choose to do so we promised them we won't board the ship - they will be able to pass the cargo to security inspection in Ashdod port and we'll pass it on to Gaza."

But the ship's crew has ignored the order and is continuing towards Gaza.

Israeli ships surrounded the Rachel Corrie about 30 nautical miles from Gaza in the Mediterranean Sea.

But so far military authorities have not boarded the ship.

The Irish-owned vessel - named after American pro-Palestinian activist Rachel Corrie, who was killed in the Gaza Strip in 2003 - has 11 passengers on board.

It is carrying tonnes of aid for Gaza, including cement, which Israel bans because of concerns it could be used to produce weapons.

Israel has offered to deliver the aid itself if it passes a security inspection, but those on board the Rachel Corrie have refused.

The passengers have said they will not resist if Israeli authorities take over the ship.

Those onboard say they are prepared for a confrontation, but that they do not have weapons on the ship.

One of the ship's crew, Derek Graham, says they have a clear plan in case they are stopped.

"There will be no resistance put up by us whatsoever," he said.

"I have instructed everybody to sit at midships - this includes the crew that are not essential to the running of the ship - make sure that they are sitting on the ground with the hands shown, the palms of their hands shown, to show they have no weapons of any kind in their hands, because we do not want [what happened] last Monday morning."

Mr Graham was referring to the incident five days earlier, when nine men were killed by Israeli commandos who boarded a Turkish ship heading for Gaza.

- ABC/Reuters/BBC/AFP