It will be the most unpredictable and hastily-prepared summit between the leaders of the US and Russia. It was touch and go that it would take place at all as rumours of the resignation of President Yeltsin swept Moscow and caused tremors in Washington and on Martha's Vineyard where President Clinton tried to take a holiday this week.

Strobe Talbott, US deputy Secretary of State, who met Mr Yelstin yesterday, later telephoned Mr Clinton to pass on the Russian leader's strong denial of the resignation rumours.

Mr Clinton must wish he had never agreed to have this summit meeting with President Yeltsin. But that was last month when the agenda was to encourage Russia to continue with economic reforms, negotiate further cuts in the nuclear arsenals of both countries and discuss NATO expansion and Kosovo.

Now Russian economic reform is past tense as the rouble collapses and as the old guard politician, Victor Chernomyrdin, takes over the government and may be poised to take over from President Yeltsin.

President Clinton himself is also in a much more precarious position than he was last month because of the dramatic developments in the Monica Lewinsky affair. A summit at this time between two struggling world leaders may not inspire much confidence.

Mr Clinton's aides are apprehensive at the prospect of the joint press conference in Moscow being dominated by questions from American media about the Lewisnky affair. Mr Clinton is already under pressure to repair the damage done by his ill-judged speech last week admitting the affair and this could be his first opportunity to do so but in an inappropriate setting.

There is dismay in Washington over the crash of the Russian economic reform programme and the dismissal of two of its main architects, former prime minister, Sergei Kiryenko, and economist, Anatoly Chubais. This dismay is deepened by the likelihood of the return of hardline communists to the Chernomyrdin government calling for more state control of banks and enterprises.

Earlier this week, President Clinton spoke to Mr Yeltsin as the rouble began its meltdown to encourage him to stick by the economic reform programme but it was too late. Now the Americans wonder what this summit will have to talk about on the economic front. It will be impossible for the US to approve measures which are seen as moving the economy back towards the Soviet era.

But as long as President Yeltsin is still in office - and this was uncertain until he denied yesterday he would resign - it is seen as better to go ahead with the summit so that some kind of links at top level can be maintained. Mr Yeltsin's condemnation of the US missile strikes at Sudan and Afghanistan still rankles here.

What President Clinton can do to help end the chaos in Russia is not clear. His advisers insist he will not be "bringing any money".

Mr Yeltsin may press Mr Clinton to use US influence to get the International Monetary Fund to release the $4.3 billion second tranche of its aid package for Russia due on September 15th. But the IMF first wants the Duma to approve economic reforms concerning tax collecting, the budget deficit and the convertibility of the rouble.

The Duma seems in no mood to agree to this. Instead it is reported to have drafted its own economic package harking back to Soviet-style controls of prices and currency.

Mr Clinton may try and use the IMF package to put pressure on Mr Yeltsin not to abandon the reform programme. But refusal by the IMF to pay over more funds could make the Russian economic situation even worse.

The summit was meant to encourage Moscow to move on the reduction of longrange nuclear missiles as agreed under the Start-2 treaty. The US Congress ratified the treaty but the Russian Duma has so far refused to do so as a way of showing Mr Yeltsin its discontent with the results of the economic reform programme.

Several months ago it looked as if Mr Yeltsin would be able to assure Mr Clinton that Start-2 would be ratified later this year and the US and Russia could move towards further missile reductions under Start-3. But now all bets are off.

A highlight of Mr Clinton's visit will be a speech directed at a younger generation of Russian business leaders and students over the heads of the present leaders. The President will, his aides indicate, try to build bridges between the US and these future leaders.

Other items on the summit agenda are the situation in Kosovo, NATO expansion and US concern over Russian co-operation with Iran on missile technology.