This week's visit to Washington by the Israeli Prime Minister, Ehud Olmert, will be scrutinised primarily for what he and George Bush have to say about Iran's nuclear enrichment programme.

But Mr Olmert will not want to limit the discussions to Iran. He will also seek demonstrative backing for his "convergence plan", which aims to unilaterally fix Israel's permanent borders with the Palestinians within the next few years.

Here is where an element of discomfort for the US may creep in. America may back Mr Olmert's ambitions in private, but is less keen to commit itself publicly to a plan that is sure to alienate Europeans, the Arab world, Muslims in general; in fact almost everybody except Israel, when their support or acquiescence is required in the conflict with Iran.

Mr Olmert has toned down his plan a little since he was elected in March, suggesting he may now consult some Palestinians - it is not clear who, exactly - before drawing lines in the sand. But the grace period will not extend for more than a few years, as the Prime Minister wants the borders fixed while Mr Bush is in the White House.

It is also unclear whether the offer to consult the Palestinians is sincere, or an alibi, designed to allow Israelis to declare they "tried" talks but now, sadly, have no alternative but to proceed in drawing borders on their own.

There may be no stopping Mr Olmert. After all, following Hamas's surprise victory in the Palestinian polls, he was effectively elected on a mandate to throw up a wall and push as many Palestinians as possible behind it.

But America would be making a strategic error if it blessed that strategy in public or private. It would be an oxymoron to describe unilaterally imposed borders as permanent. They will be no such thing. The Americans should remind their ally that without a real and sustained attempt to involve at least the Palestinians' moderate president, Mahmoud Abbas, in negotiations, any borders that Israel designs will be remain provisional. The Americans should also tell Israel it has got to start thinking more seriously about what will make a future West Bank state viable.

The European Union's representative to the Middle East peace process has described the Olmert plan as equivalent to locking a door and then throwing away the key. Dumping the Palestinians behind a wall of the kind that Mr Olmert seems bent on erecting, he says, is not the longed-for solution.

Obviously, Mr Olmert disagrees. But it would be unfortunate if he were able to boast that the US openly supported a plan which, as far as anyone can tell from its outlines, is unlikely to deliver peace or stability.