Success at Rambouillet will impose on the international community, particularly NATO, a major military obligation. Between 20,000 and 36,000 ground troops will have to police any agreement.

With the 30,000 troops currently in Bosnia already costing up to $7 billion a year, the prospect also means a massive financial commitment.

NATO insists that ground forces will only be committed if a political settlement is agreed, with planners warning that an attempt to impose agreement on still warring factions would require as many as 100,000 troops, a politically impossible commitment.

Ominously, however, Serb negotiators, although indicating their willingness to come to talks, have also adamantly denounced any suggestion of NATO or any foreign troops on the ground as an attack on their sovereignty. But observers say that a deal - assuming the talks manage to start - will not be struck without agreement on troops and suggest the Serb position may be a negotiating ploy designed to pave the way for an argument about who should actually control such troops, NATO or the UN, and what their mandate would be.

For the western powers there can be no argument on this issue. The experience of UN ineffectuality and confused command structures in Bosnia makes a UN-commanded operation unacceptable, although a Security Council mandate for a NATO-commanded peacekeeping or peace-enforcing mission will certainly be sought. The US has also insisted in the past that US troops could not serve under non-US command although they appear to be willing in this case to make an exception in the context of a NATO operation. Their likely contribution of 2,000 to 4,000 men will prove crucial to the communications, logistics and intelligence components of any mission, as well as in providing air support.

Senior US military officials have made clear to Congress this week that any larger contribution will not be forthcoming and that the US expects Europe to take the main burden, although they prepared politicians for the likelihood of a three-year engagement.

London and Paris have already indicated they are willing to contribute 8,000 and 5,000 men respectively, with Germany expected to come up with a further 3,000. The multinational force is expected also to include other EU member-states and, crucially, Serbia's "friend", Russia, as well as Polish, Hungarian and Czech forces.

In Bosnia a Muslim contribution was seen as important - Egyptians and Jordanians - and may well form part of any package agreed at Rambouillet.

Irish diplomatic sources say that there has as yet been no approach to Ireland to contribute and that even speculation about involvement would be premature. The Irish have five unarmed military officers in the current OSCE monitoring mission and 50 military police serving under NATO command in Bosnia.

NATO has confirmed that the 2,000 troops currently based in Macedonia as an emergency extraction force - known as "the dentists" - will form the core of any peacekeeping contingent in Kosovo. They are likely to be supplemented by troops from NATO's Rapid Reaction Force under the British general Sir Michael Jackson, who is expected to take overall command.

The operation, in difficult terrain, is also expected to involve a heavy commitment in tanks and artillery as well as engineering battalions.

The director of the CIA, acutely aware of US sensitivities to casualties, warned the Senate Armed Services Committee that the operation would be difficult. "The situation in Kosovo is fraught with more danger than in Bosnia where all parties had exhausted themselves," he said, warning that "force protection issues" would have to assume a high priority. "There are a lot of arms out there."

And he warned that the KLA "will emerge from the winter, better trained, better equipped, and better led than last year". Experience in Bosnia has shown, however, that US wishes to minimise casualties will also have to be weighed against the strategic need to get troops out from their tanks to deal with the sort of low-level disputes and intimidation which police forces tend to handle better.

The International Crisis Group, an NGO specialising in detailed monitoring of conflict, has been critical of the failure of NATO-led forces in Bosnia to protect returning refugees on the ground and has already called for NATO to include a significant "gendarmerie" element trained in dealing with civil disturbances and low-intensity operations in its Kosovo planning. Currently the NATO commitment is already substantial. NATO has marshalled a fleet of around 200 warplanes, mostly US fighters and bombers based in Italy, to threaten possible targets in Serbia, which has fewer than 80 at its disposal.

The NATO planes are equipped with either cruise missiles or air-to-ground assault missiles, and with air-to-air defence missiles.

The alliance decided last month to beef up its resources in the region, sending the aircraft-carrier USS Enterprise and eight other NATO warships into the Adriatic, which they should reach some time in the next few days. The air-fleet includes long-range Stealth bombers, several F-15 and F-16 fighters, and a variety of planes for purposes ranging from refuelling to radar-scrambling.

By contrast, the Yugoslav airforce has 79 Russian-built MiG warplanes as well as a broad airdefence network with surface-to-air missiles, according to the International Institute for Strategic Studies in London.

The Serbs have eight surface-to-air missile batteries at eight different sites, and their arsenal includes at least 100 surface-to-air missiles with a range of 2 to 16 km, the IISS said.