Israel has reiterated that a ship carrying aid and international activists will not be allowed to break its blockade of the Gaza Strip.
The MV Rachel Corrie is on schedule to reach the territory early on Saturday.
The confrontation comes days after Israeli commandos stormed a flotilla of aid ships approaching Gaza, leaving nine people dead and many more injured.
The raid brought strong condemnation of Israel, especially from Turkey where most of the victims were from.
"We will stop the ship, and also any other ship that will try to harm Israeli sovereignty," Israeli Foreign Minister Avigdor Lieberman told Israeli television.
"There is no chance the Rachel Corrie will reach the coast of Gaza."
Israeli Prime Minister Benjamin Netanyahu told his cabinet on Thursday that the boat would not be allowed to reach the territory.
The ship is named after a US college student who was crushed to death by an Israeli army bulldozer as she protested over house demolitions in Gaza.
Israel has told the ship to dock at the Israeli port of Ashdod and promised that the aid will be taken by road to Gaza, after officials have examined it.
In a statement, the White House reiterated its belief that the blockade of Gaza was "unsustainable", but urged activists on the Rachel Corrie to do as the Israelis asked and take the aid to Ashdod.
Irish Nobel peace laureate Mairead Maguire, who is aboard the ship, has insisted it will not be diverted.
"We are not afraid," she told Irish broadcaster RTE by satellite telephone.
"We started out to deliver this cargo to the people of Gaza and to break the siege of Gaza, that is what we want to do," the 66-year-old human rights campaigner said.
The Irish foreign ministry said the ship should be allowed to proceed to Gaza.
"Those on board the Rachel Corrie have made clear their peaceful intentions and have stated that they will offer no resistance to Israeli forces," it said.
Activists told the BBC that they aimed to arrive just outside Israel's 20-mile (32km) exclusion zone off Gaza by Saturday morning.
They said there were 20 people on board, including five Irish nationals, six Malaysians and nine crew members.
Accounts differ as to what happened when Israeli soldiers abseiled from helicopters on to the Turkish passenger ship Mavi Marmara in the early hours of last Monday.
Israel says the commandos were attacked with sticks and knives and used their weapons in self-defence. Activists say troops opened fire without provocation.
Israeli soldiers were among those injured in the incident.
Protests against the raid have taken place in cities around the world.
In Lebanon on Friday, Hassan Nasrallah, the leader of the radical group Hezbollah, addressed thousands of his supporters via a video link in Beirut saying another flotilla should be organised as soon as possible.
Threats over trade
Turkey has warned it might reduce economic and defence ties with Israel following Monday's raid, which took place in international waters.
Deputy Prime Minister Bulent Arinc said Ankara was "assessing deals with Israel", until now a close ally.
Prime Minister Recep Tayyip Erdogan ratcheted up the pressure on Friday, saying he did not view Hamas, which controls Gaza, as a terrorist organisation.
In a televised speech Mr Erdogan described Hamas as "resistance fighters who are struggling to defend their land".
Hamas is designated a terrorist organisation by Israel, the US and the EU.
Israel and Egypt imposed the blockade after Hamas came to power. Egypt has occasionally opened its Rafah crossing with Gaza on humanitarian grounds, and opened it again following the Israeli sea-borne raid.