'Whether the engagement succeeds or fails, the US is prepared to offer assistance to governments to mitigate the consequences of any satellite debris impact on their territory,' said Ms Christina Rocca, the US ambassador to the treaty-negotiating body, yesterday.

The US has been advising countries of problems with the bus-sized satellite loaded with toxic fuel, officials said, but Ms Rocca stepped up the warnings by informing the conference of the shoot-down decision disclosed in Washington on Thursday.

US President George W. Bush has authorised the Pentagon to attempt the shoot- down of the inoperable National Reconnaissance Office satellite, which is currently in a decaying orbit.

Satellites have de-orbited before, but never with this much associated risk, said US Marine Corps General James Cartwright, vice-chairman of the Joint Chiefs of Staff.

Officials have compared the effects of hydrazine fuel to those of chlorine or ammonia.

'It affects your tissues and your lungs - it has the burning sensation,' said Gen Cartwright. 'If you stay very close to it and inhale a lot of it, it could in fact be deadly.'

The military hopes to smash the satellite as early as next week, just before it enters Earth's atmosphere.

The US has recently modified three missiles and three navy ships to perform the mission from the northern Pacific Ocean. If the shoot-down fails, the satellite is expected to make an uncontrolled re-entry into the atmosphere around March 6.

'At present, we cannot predict the entry impact area, which could occur in any region on Earth's surface between 58.5 deg north and 58.5 deg south latitudes,' Ms Rocca said.

That area includes most of the inhabited world.

However, analysts have voiced scepticism about the explanation for the dramatic move. They said the decision raises new concerns about an arms race in space and could drive Russia and China to respond.

The Pentagon, they argued, is more likely testing its ability to target other states' satellites and overstating the toxic fuel threat - a suggestion rejected by US officials.

The US military and intelligence agencies rely extensively on space-based technologies, which provide capabilities ranging from communications and positioning data to precision targeting in counter-terrorism operations.

That dependence, coupled with concern about China's test last year, has driven Washington to demonstrate its abilities to hit satellites in space, security experts said.

Last January, China launched a ground-based missile into an obsolete weather satellite - drawing international criticism and worries inside the Pentagon that Beijing now has the ability to target critical military assets in space.

Now the Pentagon is 'going to use this as a test of an anti-satellite system to destroy the satellite', said security expert Ivan Oelrich of the Federation of American Scientists.

'I'm concerned about the implications this will have with the Chinese and the Russians for starting an anti-satellite arms race, which will do nobody any good but will particularly threaten the United States because we are far and away the biggest presence in space,' he said.

Arms control specialist Jeffrey Lewis of the New America Foundation warned that China would cite the intercept to justify its anti-satellite test last year.'The politics are terrible,' he said.

The Pentagon manoeuvre will be the first time the US has conducted an anti-satellite operation since the 1980s, analysts said.

There also is concern in some quarters that the debris could reveal US national security secrets if recovered by other nations, said a report in the Aviation Week magazine.

ASSOCIATED PRESS, REUTERS, NEW YORK TIMES

ARMS RACE WORRY

'I'm concerned about the implications this will have with the Chinese and the Russians for starting an anti-satellite arms race, which will do nobody any good but will particularly threaten the United States because we are far and away the biggest presence in space.'

SECURITY EXPERT IVAN OELRICH of the Federation of American Scientists