A senior Washington administration official told journalists on Saturday President George Bush would hold Sharon to a pledge he made in their first ever meeting three years ago.
"We consider a pledge, a pledge."
Earlier, the Israeli PM said he told Bush last week Tel Aviv was no longer bound to keep Arafat alive.
Speaking on Israel's Channel Two, Sharon also confirmed in his first meeting with President Bush three years ago that he would not harm Arafat "physically".
"But I am released from this commitment. I release myself from this commitment regarding Arafat."
With no freedom of movement, Arafat has lived with death and the possibility of his Ram Allah compound being stormed on a daily basis.
"I am not afraid of Sharon's threats. He has a history of attempting to target me," he told an Arab member of the Israeli Knesset, Ahmad Tibi.
Arafat: Accustomed to Israeli
threats in besieged Ram Allah HQ
Israel accuses Arafat of encouraging Palestinians to resist occupation.
Sharon's comments about Arafat come just over a week before a crucial party vote within his Likud party on a unilateral plan for a withdrawal from Gaza.
In a move condemned by Palestinian leaders President Bush gave his support to 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.