Israeli ships are reported to be rapidly approaching the Freedom Waves flotilla bound for Gaza.
Dr Fintan Lane - who is aboard the Irish ship MV Saoirse - sent an urgent radio message to the Irish Ship to Gaza team in Dublin a short time ago.
The ship, along with Canadian boat the Tahrir, is now 48 nautical miles from Gaza's shores.
The Canadian vessel has received a radio message from the Israelis requesting its final destination.
Irish Ship to Gaza is calling on the Government to issue an urgent statement to the Israeli authorities warning against any harm to the passengers aboard the MV Saoirse.
Israeli Military spokesperson Avital Liebovitch has confirmed that the Israely navy made contact with the vessels, notifying them that they can return or sail to an Egyptian port.
The Israeli Defence Forces have ordered the navy to board the vessels if they refuse, but this has not happened yet, Lt Colonel Liebovitch said on Twitter.
Communications from the vessels meanwhile appeared to have halted.