The bombers flew in pairs to Fairford, a Royal Air Force base in Gloucestershire, 120 km west of London.

``This is a warning,'' said Col Wendell Griffin, detachment commander, after the aircraft landed. ``We are glad to be here and we are very prepared,'' he added. ``... But we are on standby, and we will be ready and able to fly missions eight or nine hours from now.'' He said flying time from Fairford to Yugoslavia would be between four and 12 hours depending on the route taken.

A seventh B-52 which set out on the 6,500-km journey was a spare and turned back, as is customary, to Barksdale air base, Louisiana, said Major Robyn Grantham of the US Air Force.

The B-52s are among 260 US aircraft, poised to take part in strikes which NATO plans to authorise tomorrow unless there is a last-minute dealwith Yugoslav president Slobodan Milosevic over the ethnic Albanian majority in Kosovo.

``There is a way out and the way out for Mr Milosevic is to comply with the will of the world community, the objectives of the UN Security Council and the United Nations,'' said defence secretary George Robertson, speaking on Sky TV. ``If he does that ... then he can avoid the use of force, if not, he is on the very last countdown and the B-52s arrive today.'' Robertson added that NATO was determined to act before the winter because of the plight of thousands of refugees living in the open after being driven from their homes.

Meanwhile, in Moscow, a Russian defence ministry official warned that Russia could consider large-scale military cooperation with Belgrade in the event of air strikes against Yugoslavia. ``Russia would then have the right to large-scale military cooperation with Yugoslavia,'' said Leonid Ivashov, head of department for military cooperation in the defence ministry. ``If standards of internationallaw were to be violated, then the embargo imposed in March 1998 will also cease for us.'' ``We cannot abandon a brotherly people in a situation like that,'' Ivashov said.