The United States has insisted that Israeli Prime Minister Ariel Sharon honours a promise not to harm the Palestinian leader, Yasser Arafat.
Arafat: No stranger to Israeli threats
Earlier, Mr Sharon said he had told President George W Bush at talks last week that he would no longer be bound by a pledge made three years ago.
Mr Arafat himself has said he was not afraid of the Israeli prime minister's threat.
Israel accuses Mr Arafat of supporting Palestinian militants.
Mr Sharon told Israel's Channel Two that in his first meeting with President Bush three years ago "I accepted his request not to harm Arafat physically.
"But I am released from this commitment. I release myself from this commitment regarding Arafat," he said.
A senior Bush administration official was quoted by Reuters as saying: "We have made it entirely clear to the Israeli government that we would oppose any such action and have done so again in the wake of these remarks.
"We consider a pledge, a pledge."
Mr Sharon's comments about Mr Arafat come just over a week before a crucial party vote within his Likud party on his unilateral plan for a withdrawal from Gaza.
President Bush gave his support to Mr Sharon's proposal describing it as "historic and courageous".
But in the last month, Israel has killed both the spiritual and political leaders of Hamas in missile strikes on Gaza.
The BBC's Rob Watson says that the US was widely criticised in the Arab world and beyond for its failure to condemn those attacks and is clearly in no mood to face another diplomatic storm should Yasser Arafat be targeted next.
After Mr Sharon's comments, Mr Arafat remained defiant.
"I am not afraid of Sharon's threats. He has a history of attempting to target me," he told an Israeli Arab member of the Knesset, Ahmed Tibi.
At the beginning of April, Mr Sharon earned a rebuke from the US after he warned Mr Arafat not to "feel immune".
And on Friday the US State Department said it remained opposed to any attempt to assassinate Mr Arafat.
"Nothing has changed in the US position," spokesman Richard Boucher told reporters.