It would confound the international campaign to curb Iran’s nuclear ambitions by restricting its ability to make bombs through the alternative method of enriching uranium.

The risk is viewed with such gravity in Washington that the United States has launched a concerted diplomatic and covert effort to prevent it, according to diplomats based in Pyongyang and Beijing.

The belief that Iran and North Korea are talking about plutonium stems from a recently reported offer of oil and gas from Tehran in exchange for nuclear technology.

The discovery by the International Atomic Energy Agency (IAEA) in 2004 that North Korea had sold an estimated 1.7 tons of uranium to Libya established a precedent for the sale and showed how hard it is to stop, diplomats say.

The Americans were aghast to learn last year that while engaging in disarmament talks, North Korea had made enough plutonium to amass a stockpile of about 43 kilograms, perhaps as much as 53kg. For the first time since the nuclear crisis began in 1994 it has sufficient fissile material to sell some to its ally while retaining enough for its own purposes.

Plutonium is the element used to fuel the bomb that destroyed Nagasaki in 1945. Between 7kg and 9kg are needed for a weapon. According to Siegfried Hecker, the eminent American nuclear scientist, officials in North Korea intend to restart a reactor that will produce 60kg a year.

Iran already has a nuclear programme devoted to plutonium research, according to John Bolton, US ambassador to the United Nations. In a 2004 speech Bolton said the Iranians were building a research reactor “optimal for the production of weapons-grade plutonium”.

Making a plutonium bomb can be more complicated than making a uranium bomb, but Iran and North Korea work well together on military science. The Americans believe Iran is sharing data on missile tests with North Korea in exchange for nuclear technology.

The US State Department revealed last summer that 11 shipments of nuclear materials bound for North Korea and Iran had been intercepted under the proliferation security initiative, in which 60 nations including Britain co-operate in air and sea searches. It refused to disclose any details.

Alarm bells sounded again in Washington late last autumn after nuclear disarmament talks in China ground to an inconclusive and ill-tempered halt. Christopher Hill, the American negotiator, came out of a meeting to tell colleagues that “those f***ers say they’re going to go right ahead and build nuclear weapons no matter what we do”, according to an official who overheard the remark.

In November western intelligence sources told Der Spiegel, the German news magazine, of a clandestine visit to Pyongyang by an unnamed high-ranking Iranian official who offered North Korea a huge amount of oil and natural gas in exchange for help on nuclear research and missiles.

Sources in Tehran say Iran’s Revolutionary Guards recently established their own institutional links with North Korea, bypassing traditional policy channels. “Whatever they’re up to, it’s probably done through the Revolutionary Guards,” commented a western diplomat.

Last week the Americans were pushing their campaign to strangle North Korea’s illicit fundraising through currency and cigarette counterfeiting, drug smuggling — including one speciality, fake Viagra — and weapons sales.

Condoleezza Rice, the US secretary of state, is due to meet foreign ministers from Britain, France, Germany, Russia and China in London tomorrow to discuss their differences on how to stop Iran going nuclear. Rice hopes that a meeting of the IAEA on Thursday will resolve to refer Iran to the UN security council as a prelude to sanctions.

The United States is also mounting a diplomatic offensive to get the message across, through China and South Korea, that a transfer of plutonium would cross a political red line.

One wild card remains the ability of Kim Jong-il, the North Korean leader, to take a gamble like Colonel Muammar Gadaffi of Libya, who abandoned his nuclear programme in exchange for international acceptance.

Back in Pyongyang, diplomats are left guessing as people enter and leave the Iranian embassy. One fact, however, is known. Jalaleddin Namini Mianji, Iran’s ambassador who was appointed by the previous “reformist” government, is being recalled. His successor’s credentials are certain to reflect the hard line of President Mahmoud Ahmadinejad, who threatened to wipe Israel off the map.

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