This week has seen an intensification of diplomatic activity exploring the chances of a political settlement.

Most of the activity is centred on the Russians. Viktor Chernomyrdin, President Yeltsin's envoy, has already held talks this week in Germany and Italy; and he plans to go to the UK and France after his visit to Belgrade.

Mr Chernomyrdin says he has concrete proposals, but there is little sign they can breach the huge gap between Nato and Yugoslav President Slobodan Milosevic.

The language used by the senior American envoy, Strobe Talbott, was not encouraging. We're working with the Russians and the United Nations, he said, to keep alive the possibility of a political settlement.

But there is a lot of very, very hard work to be done. A British official expressed pessimism about Mr Chernomyrdin's chances and said it was difficult to discern exactly what he had in mind.

Keeping Russia and Nato happy

According to Mr Talbott, the problem is identifying a settlement that would square both with Nato's bottom line and with the position and objectives of Russia.

Strobe Talbott: Working to keep possibility of a deal alive

Essentially, the western powers want Moscow to adopt their demands and then try to get Mr Milosevic to accept them. But the Russians refuse simply to act as a messenger boy.

The first obvious stumbling block is Russia's - and Belgrade's - demand that the Nato air strikes should stop first. As Mr Chernomyrdin put it, it is useless trying to resolve the problem under bombs.

Nato says the Serbs have to withdraw their forces from Kosovo - or at least start doing so - before it will talk about a halt to the bombing. This is one of these familiar negotiating deadlocks that can sometimes be broken.

Peacekeeping force: Armed or not?

But the central disagreement is over the deployment of an international force in Kosovo to ensure that the Albanian refugees can return home.

The nature of this force is clearly at the heart of Mr Chernomyrdin's mission.

Russia and Belgrade want Nato strikes to stop first

President Milosevic seems prepared to accept only an unarmed body, not a military one, which could hardly be described as a force at all.

The Russians have been leaving this aspect publicly vague, although the UK Foreign Secretary, Robin Cook, says they do now accept that a military force will be essential.

Mr Chernomyrdin may be calculating that he may be able to sell a military force in Belgrade if it does not contain Nato troops or at least is not dominated by Nato. But for the West, this is essential: only that kind of force would be sufficient to reassure the Kosovo Albanians and make sure there is no Serb resistance.

UN's role

The Russians are emphasising the importance of the United Nations in any solution. They are proposing a UN force, in line with their consistent stress on the role of the Security Council.

The West would be happy enough with Security Council authorisation - that is partly why they are working with Russia, because a Russian veto would block it.

On the other hand, a force under UN command would bring back all the unhappy memories of Bosnia before Nato got involved there.