Organisers of the Gaza flotilla said they lost contact with the Gaza-bound MV Rachel Corrie on Friday just as they are seeking to delay the latest bid to bust the embargo with an aid-laden ship.
The ship had been on course for arrival in the Palestinian enclave on Saturday, just five days after Israeli commandos killed nine activists aboard a Gaza-bound aid flotilla in a botched raid that plunged Israel into a diplomatic crisis.
"The situation is we lost all contact with the boat. We assume this was sabotage by the Israelis," said Audrey Bomse of the Free Gaza Movement.
It was now unclear whether the Irish and Malaysian activists aboard the ship would turn around or steam on towards the Hamas-run Gaza which is under a crippling blockade which Israel says is aimed at halting Palestinian rocket fire.
Monday's raid sparked worldwide outrage with more massive protests expected on Friday, particularly after weekly prayers in Muslim countries.
In Kuala Lumpur, some 5000 Malaysians rallied outside the US embassy where the Israeli flag was burned.
'Allah will destroy you Israel'
Some demonstrators burned the Israeli flag while others brandished posters that said "Destroy America, Destroy Israel ? Long Live Islam" and "Allah will destroy you Israel".
Hezbollah chief Hassan Nasrallah called for a mass rally in Beirut on Friday evening where he said he would announce "serious measures."
In Jerusalem, police restricted access to the flashpoint Al-Aqsa mosque compound and deployed in force in and around the Old City.
Angry anti-Israel protests have been staged across the Middle East and in major cities since Monday's deadly raid, with vast crowds taking to the streets to demand an end to Israel's blockade of the Gaza Strip.
A massive rally was staged on Thursday in Turkey ? a key Israeli ally ? whose activists played a major role in the six-ship flotilla and whose president warned that ties with Israel "will never be the same" after the attack.
Turkey sent two medical planes to Israel early on Friday to bring back five of its nationals wounded during the assault on the flotilla in which eight Turks and a US national of Turkish origin were killed, the Anatolia news agency said.
Israel has warned it will also stop the blockade-busting bid by Rachel Corrie ? a 1200 tonne cargo ship named after a US activist killed in 2003 as she tried to prevent an Israeli bulldozer from razing a Palestinian home.
"As a result of these threats, we're going to pull Rachel Corrie into a port, add more high-profile people on board, and insist that journalists from around the world also come with us," the Free Gaza movement said.
But Bomse later said the decision couldn't be communicated to those aboard the vessel, who include Irish Nobel Peace laureate Mairead Maguire (66).
"We're hoping communications get turned back on so we can inform them of the decision," Bomse told AFP.
'The most serious consequences'
On Thursday afternoon, organisers said the Rachel Corrie was about 250 miles (400 kilometres) from the spot in international waters where the six boats were boarded on Monday.
Irish Prime Minister Brian Cowen has said that the ship must be allowed to reach Gaza and warned of "the most serious consequences" if Irish citizens are injured.
The US administration has so far refused to explicitly single out the Israeli government for blame.
Palestinian leader Mahmud Abbas will ask President Barack Obama to make "bold decisions" on Middle East peace when the two meet in Washington on Wednesday. He will first travel to Turkey to pay his condolences.
The Israeli authorities and the activists had conflicting versions of what happened during Monday's pre-dawn raid.
Bulent Yildirim, head of the Islamic charity Foundation of Humanitarian Relief, which spearheaded the Gaza aid fleet, said Israeli soldiers fired indiscriminately when they stormed the Turkish ferry Mavi Marmara.
He said activists used iron bars against the Israeli forces "in self defence". They also seized the soldiers' weapons but threw them in the sea, he added.
Israel has said the commandos only opened fire after they came under attack with clubs, knives, guns and other weapons. It said two pistols taken from soldiers were found, their magazines empty.
Israel rejected a bid by the UN Human Rights Council to set up an investigation.