The Israeli navy was preparing to stop a Libyan aid ship that was closing in on the Gaza Strip coast on Tuesday, the military said as activists on board said they had been warned to alter course.
"Israeli authorities have given us until midnight on Wednesday (2100 GMT) to change course and head to the (Egyptian) port of El-Arish," Mashallah Zwei, an activist on board said by satellite phone.
"Otherwise they are threatening to intercept the boat with their navy," said Zwei, a member of the Kadhafi Foundation charity which organised the shipment, adding that the crew had informed the navy it would study the request before responding.
A military spokesman denied they had given the vessel an ultimatum, saying that they only gave them "a clarification about what they already knew, that they could not go to Gaza."
"The navy has begun preparations for stopping the ship, should it attempt to violate the naval blockade," he told AFP. "We are now making contact with them."
Fears of a new and potentially deadly standoff came exactly six weeks after Israeli commandos launched a pre-dawn operation to prevent a flotilla of aid ships from breaching its naval blockade on Gaza.
In the resulting fracas, nine Turks, including a dual US national, were shot dead and dozens of other people injured, including nine Israeli commandos.
As news broke of the military's contact with the vessel, public radio broadcast a crackly snippet of what it said was exchange between the navy and a person aboard the Almathea, which was picked up by the station's monitor of foreign broadcasts.
"No weapons, no guns, no weapons on board," said a voice in heavily-accented English to a background of radio static.
Zwei said the navy had "threatened to send their warships to intercept the boat and escort it toward the (southern Israeli) port of Ashdod if we do not change course.
"We explained to the Israeli authorities that our original destination was Gaza and that we are not here for a provocation," he said.
"We also specified that we are transporting only foodstuffs and medicines and we asked them to let us discharge our cargo in Gaza."
The 92-metre freighter left a Greek port on Saturday and was expected to arrive off Gaza's territorial waters on Wednesday, said the group behind the mission, a charity run by Seif al-Islam, son of Libyan leader Moamer Kadhafi.
The latest standoff came a day after the Israeli military published the results of an internal inquiry into the May 31 raid, which found that while mistakes had been made, the troops' use of live fire during the operation was "justified."
The report also made a point of saying that no country in the world had ever managed "to stop a vessel at sea in a non-hostile manner."
Over the last week, Israel has made a flurry of diplomatic efforts to try and convince the boat, named the Amalthea, to change course and deliver its cargo of 2,000 tonnes of foodstuffs and medicine to El-Arish.
As well as diplomatic channels, pressure was also being exerted upon the Amalthea's owner and its captain to change course, the Kadhafi Foundation said.
"Pressures are escalating at various levels on the owner of the vessel and its captain to force the ship to change its course and not to go to Gaza port," the foundation said on its website.
"The foundation has received a letter from the company which owns the vessel confirming sustained pressures on them up to now. The ship's owner confirmed he had rejected the pressure; however, he stressed that he did not intend to enter into any confrontation."
In Cairo, an official confirmed the authorities had received a request for the Libyan ship to dock in El-Arish, near Egypt's border with Gaza, but did not specify the origin of the request.
On Monday, EU foreign affairs chief Catherine Ashton urged both parties to avoid any "unnecessary confrontations."
"I am concerned about reports of further ships carrying humanitarian aid to Gaza. I would like to call on all of those involved to exercise calm and restraint at this particular juncture," a statement from her office said.
"An escalation of tensions and unnecessary confrontations should be avoided."
Global pressure over the May 31 debacle has since forced Israel to significantly change its policy on Gaza, and now it only prevents the import of arms and goods it says could be used to build weapons or fortifications.