At a Ministry of Defence briefing on Thursday, UK Director of Operations Air Marshal Sir John Day said allied forces had "increased the use of all-weather weapons such as cruise missiles".

The news comes a day after the Pentagon was forced to admit current stocks of the cruise "won't last forever".

Defence chiefs in the US have announced a plan to begin converting nuclear cruise missiles to carry conventional warheads.

But it is unclear how long the process will take, with some estimates claiming 12 to 18 months. There is no up-and-running production line for new cruise missiles.

A cruise missile launched from the USS Gonzales in the Adriatic Sea

One newspaper report said the US Air Force spent at least a fifth of its available stock of 150 cruise missiles with conventional warheads in the first week of attacks on Yugoslavia.

The Times report also says that the US navy's much larger supply is dwindling even faster.

But independent defence experts differ in their opinion of how serious the shortage might be for the Alliance.

Pentagon spokesman Ken Bacon said there were currently "ample supplies" of all air- and sea-launched cruise missiles. But the "most limited" were of conventional air-launched cruise missiles, "we do have enough to continue striking high-value targets".

Month's supply left?

But with no end in sight to the bombing campaign, it is thought that at the present rate of striking the USAF could use up all its remaining cruise missiles before the end of the month.

The air force said it had received White House approval for a $51m programme to convert 92 nuclear-tipped cruise missiles into conventional missiles, and fit a new guidance system. However, this still has to be ratified by Congress.

The B-52 can carry 20 cruise missiles

Meanwhile, the US Navy, which has more than 2,000 cruise missiles, is seeking an emergency appropriation of $113m to upgrade 324 to the latest model.

The cruise, a low-flying strategic guided missile, is described as the weapon of choice for politicians who want to keep the conflict at arm's length. A conventional air-launched cruise (CALCM), can be fired with a 3,000lb warhead 1,500 miles from its intended target.

Converting the cruise to a conventional missile first started in the mid-1980s, when the job was undertaken by Boeing. Then it was 12 to 18 months before the first modified missiles were operational.

Spent in Gulf

Modification ceased as the US began to develop a newer, less expensive missile. But this is not expected to be operational until 2001 or 2002, said Mr Bacon.

Stockpiles of the cruise were depleted during the December air strikes on Iraq, when some 400 were fired by the American military.