Israeli war ships are "shadowing" a Malaysian-funded Irish aid ship headed towards Gaza, activists on board have said.
The MV Rachel Corrie's radar was jammed as the ship sailed closer to restricted waters off the Gaza Strip early on Saturday, a spokeswoman for the campaign group supporting the ship said.
Activists aboard Rachel Corrie are attempting to break the siege of Gaza imposed by Israel, five days after Israeli troops violently intercepted a flotilla of aid ships carrying humanitarian aid for the territory, killing nine activists.
Mairead Maguire, who is aboard the aid ship told Al Jazeera just before radio contact was lost, that navy vessels were following Rachel Corrie and its radar systems have been jammed. She, however, added the situation on board was calm.
"We are not afraid and we are all advocating non-violence… and we will just sit here and go if they insist on commandeering our boat and forcing us into Ashdod," she said.
The Israeli military said that the ship was pressing ahead towards Gaza, ignoring four instructions to dock at the Israeli port of Ashdod.
It warned the ship that it was "approaching an area of hostilities which is under a naval blockade. The Gaza area, coastal region and Gaza Harbour are closed to all maritime traffic."
Avital Leibovich, an Israeli military spokeswoman, told Al Jazeera that the military had a government directive to make sure the ship does not reach Gaza.
"We are here to make sure that those instructions are followed and the ship is stopped," she said, adding that boarding the ship was an option.
Al Jazeera's Nicole Johnston, reporting from Gaza, said that nobody had heard anything in the last couple of hours.
"Last we heard that the ship was 25 nautical miles (46.3km) from the Gaza coastline," she said.
Our correspondent said that the ship had hundreds of tonnes of aid, including medical supplies, wheelchairs, cement, building materials and even note-pads for children.
Dennis Halliday, the former UN assistant secretary-general who is also on board the ship, said on Friday that they expected Israeli military to intercept them.
"The cargo was checked three times - by trade unions in Ireland, by a member of the Irish senate, as well as Irish customs at the port," Halliday said.
"The cargo is sealed and we can't even access it ourselves," he said, explaining why they refused to take the ship to Ashdod.
The Malaysian-funded ship is carrying 11 activists, including Mairead Corrigan, a Nobel Peace laureate and eight crew members.
The ship is named after an American womanwho was killed by an Israeli military bulldozer in the Gaza Strip in 2003, while trying to protest a house demolition.
The activists say they are determined to continue sailing to Gaza with their cargo of medical and construction supplies.
Those on the ship have said they will offer no resistance if Israeli forces decide to board the vessel.
The Rachel Corrie is funded by Perdana Global Peace Organisation, a Malaysian non-governmental organisation headed by Mahathir Mohamad, the country's former prime minister.
Meanwhile in Washington the US said Israel's blockade on Gaza was unsustainable.
"We are working urgently with Israel, the Palestinian Authority, and other international partners to develop new procedures for delivering more goods and assistance to Gaza," Mike Hammer, a spokesman for the White House National Security Council, said.
"The current arrangements are unsustainable and must be changed. For now, we call on all parties to join us in encouraging responsible decisions by all sides to avoid any unnecessary confrontations," Hammer said in a statement.
"It remains a US priority to provide assistance to the people of Gaza," Hammer said.
"In the interest of the safety of all involved, and the safe transmission of assistance to the people of Gaza, we strongly encourage those on board the Rachel Corrie and other vessels to sail to Ashdod to deliver their materials to Gaza," Hammer said, referring to the Israeli port.