Defence Secretary William Cohen confirmed that the reserve forces were to be mobilised.

Defence officials say as many as 33,000 reservists could be involved.

Tom Carver: Reservists will provide back-up for fighters

Most will be used to fly and service the extra 300 aircraft which Nato command has requested.

The US call-up will be the biggest since the 1991 Gulf War, when more than 250,000 reservists were mobilised.

Campaign could last months

Washington has meanwhile signalled that Nato's air strikes against Yugoslavia could stretch into the middle of the year.

General Henry Shelton, chairman of the Joint Chiefs of Staff, admitted that the possibility of US casualties in the conflict was "very real and high".

Mr Cohen acknowledged that the campaign could continue for "many, many, many weeks or even months".

The two men were testifying before the Senate Armed Services Committee.

No ground troops

William Cohen and General Shelton said the strikes could last for months

Despite the US administration's commitment to military action, it still refuses to consider ordering ground troops to move against Yugoslav forces.

Many members of the Armed Services Committee doubted that Nato could win without the use of ground troops.

But Mr Cohen and General Shelton urged the committee to give the air campaign, now in its fourth week, more time to stop President Milosevic's fight against the Kosovo Albanians.

"The reason that we have gone forward, as we have with an air campaign, is that there was not a consensus in the Nato alliance to do anything but this," said Mr Cohen.

Costs of war

With no end in sight to the conflict, American legislators are concerned at how much the Kosovo conflict will cost the US and how the bill is to be paid.

The downed Stealth fighter cost $45m to manufacture

The White House and Congress are sparring over the scope of an emergency spending bill to pay for the conflict.

The air strikes alone are reported to have cost the US $600m in the first three weeks. Congressional budget analysts say that the campaign will cost an extra $1bn a month if the strikes are sustained.

Each cruise missile fired on Yugoslavia costs between $1m and $2m, and the price tag on the only plane so far confirmed lost during the campaign - an F-117 stealth fighter - is $45m.

Administration officials had suggested the bill would be around $4bn, but there are now reports that that sum will probably be closer to $5bn.

That figure will also include direct humanitarian aid to refugees and assistance to countries in the Balkans that are giving them shelter.

US foots the bill

Nearly 80 extra US warplanes are set to join Nato's campaign. Approval is also expected for the deployment of another 300.

This would bring Nato's total air power to 1,000 aircraft - for which the US would be footing two-thirds of the bill.

Despite the substantial costs, experts say it is important to keep the figures in perspective.

Professor Ron Smith, an expert in defence economics at London University says: "In the context of a $300bn defence budget in the US, or a £25bn defence budget in the UK, they're not large numbers. There doesn't seem to be an immediate financial problem of financing the war."

But if Nato sent in ground troops, the figures would increase dramatically, he says, and further financial provisions would be necessary.

Reconstruction

BBC Economics Correspondent Andrew Walker says that while the US and the UK might be able to afford the extra expense, other Nato members, such as France Italy and Germany - with strong budget deficits - could be more stretched.

However, the biggest costs to Nato countries could come if they got involved in reconstructing Yugoslavia after the bombings had ceased.