The United States was thrown on to the defensive as the UN nuclear disarmament talks began yesterday by allegations that it had installed a new anti-missile radar in northern Norway, a few miles from the Russian border.

Moscow has denounced the installation, believed to be the worlds most advanced tracking and imaging radar, as a covert step towards the controversial US plan to develop a shield against incoming missiles, and therefore a potential breach of the 1972 anti-ballistic-missile (ABM) treaty.

The UN secretary general, Kofi Annan, speaking at the talks, said the threat of nuclear war remained "a very real, and very terrilying possibility" at the beginning of the 21st century.

He warned that the growing pressure to deploy national missile defences "could well lead to a new arms race". Mr Annan referred to the ABM treaty as the cornerstone of strategic stability and called for "great care" before taking steps that "may well reduce, rather than enhance, global security".

The Russian foreign minister, Igor Ivanov, said in yesterdays New York Times: "Every one should be aware that the collapse of the ABM treaty would have a destructive domino effect for the existing system of disarmament agreements. We would be back in an era of suspicion and confrontation."

The Norwegian government said the radar, in the border village of Vardo, was designed to keep an eye on potentially dangerous space debris, but that explanation was derided as implausible by several independent scientists.

The revelations emerged as talks began at the conference on the 1968 nuclear non proliferation treaty (NPT) at the UN's New York headquarters and US-Russian talks on a new disarmament treaty Start III, entered their second week in Geneva.

The US and Russia have both been under fire from disarmament watchdogs for negotiating in bad faith. Roth have opted to stockpile and upgrade - rather than destroy the warheads they have removed from missile silos.

When a Norwegian journalist, Inge Sellevag, asked the Norwegian government the purpose of the radar installation, he was told it was to be used by Nasa to monitor "space junk". But Nasa knew nothing about Vardo. John Pike, director of the Space Policy Project at the Federation of American Scientists, said yesterday: "One of the standard parts of creating a cover story for an intelligence operation is that the story is plausible and this cover story was not.

"This is a type of radar that was developed as part of the national missile defence (NMD) network, and I assume the reason they put it up there is to monitor Russian missile testing".

The proposed NMD system is a successor to Ronald Reagan's Star Wars scheme. It is intended to create a nuclear umbrella by co-ordinating an array of satellites, radars and missiles which would track and intercept any incoming missiles. Such a system is banned by the ABM treaty.

Tests on NMD technology are still under way and President Clinton has yet to give the system a green light, but the Yardo radar - along with the proposed upgrading of the early-warning radars - Fylingdales in Yorkshire and Thule in Greenland - suggest that the Pentagon is already committed to the strategy.

Critics of the system believe the placing of the radars confirms that its main target is Russia, not rogue states like Iran, Iraq and North Korea as US officials have assured their counterparts In Moscow. The Ministry of Defence's claim that the Fylingdales radar was aimed at monitoring North Korea were dismissed by Mr Pike, who said: "Last time I checked England was on the other side of planet from North Korea. It might be a good place to hide from North Korea but not to watch it."

Meanwhile, US plans to up-grade its nuclear stockpile and develop "new nuclear options for emergent threats" were revealed in energy department documents made available to a court after a legal challenge by disarmament and environmental groups.

Greg Mello, director of the Los Alamos Study Group, said they revealed "a shocking disregard for US commitments, especially those enshrined in the NPT, to end t

(See also article "Antimissile Front In The Northern Norway" by Theodore Postol and Anatoli Diakov).