The leaked US cables reveal the constant, largely unseen, work by American diplomatic missions around the world to try to keep the atomic genie in its bottle and forestall the nightmare of a terrorist nuclear attack.

The leaked cables tell hair-raising tales of casks of uranium found in wicker baskets in Burundi, a retired Russian general offering to sell "uranium plates" in Portugal, and a radioactive Armenian car on the Georgian border.

As part of what the US government calls its "second line of defence", it is America's diplomatic corps who are called out in the middle of the night when radiation detectors goes off on a border crossing or smugglers turn up with fissile or radioactive materials in his pocket.

Each time that happens, and UN data suggests it has happened about 500 times in the past 15 years, it means the "first line of defence" has already been breached. The fissile material (the fuel for a nuclear warhead) or radioactive isotopes (which emit harmful radiation), have already been stolen from their source.

Three months after taking office, Barack Obama vowed to secure all the world's vulnerable nuclear stocks within four years in a global drive to pre-empt nuclear terrorism. But a cash-strapped Congress has yet to do approve any increase in funding for the ambitious project and Obama's deadline looks almost certain to be missed. Meanwhile, from Africa to the former Soviet Union, there are signs it may already be too late.

In June 2007, the US embassy in Burundi reported an approach by a local elder alerting the Americans to a cache of uranium in a concrete bunker over the border in the Democratic Republic of Congo (DRC). He was concerned that it would fall into the hands of "the wrong people", specifically the Arabs who will "destroy" people with it. At the request of the sceptical Americans, he returned a few weeks later with a Congolese smuggler who said he found the material hidden at an old Belgian colonial building. He had pictures of a wicker basket with a uranium cask inside, apparently the property of the country's Atomic Energy Commision.

There was good reason for alarm. After Mobutu Sese Seko seized power in the mid-1960s, two uranium fuel rods from a colonial-era research reactor went missing. One turned up in 1998 when the Sicilian mafia were caught in a sting operation trying to sell it for over $12m (£7.7m) to a customer in the Middle East. The other is still unaccounted for.

Another decade on, security at the Kinshasa nuclear research centre had scarcely improved. A diplomatic cable in September 2006 describes the security measures separating the reactor from the university next door.

"The fence is not lit at night, has no razor-wire across the top, and is not monitored by video

surveillance," a US diplomatic team reports. "There is also no cleared buffer zone between it and the surrounding vegetation. There are numerous holes in the fence, and large gaps where the fence was missing altogether. University of Kinshasa students frequently walk through the fence to cut across [the reactor site] and subsistence farmers grow manioc on the facility next to the nuclear waste storage building."

At the same time, there was concern over established smuggling routes shipping both uranium fuel and raw uranium ore abroad, possibly to Iran. Congo has some of the richest uranium reserves on earth, both in terms of the scale of the deposits and the purity of its ore.

It would require just a few lorry loads of Congolese ore to process and enrich enough uranium for a bomb. Both Washington and inspectors at the International Atomic Energy Agency (IAEA) have been concerned that such a black market source of high quality ore, outside IAEA controls, could be used in a covert weapons programme in Iran.

A cable from Tanzania in September 2006 passed on a tip that some of the smuggled material may be passing through the nation's capital.

"According to a senior Swiss diplomat, the shipment of uranium through Dar es Salaam is common knowledge to two Swiss shipping companies ... though no one at either company would admit it in writing."

The other major front in America's "second line of defence" runs around the edge of Russia's borders, where the collapse of the Soviet Union created a black market in nuclear and radioactive material that endures two decades on.

As in Africa, the fears are based on poor security, particularly in the immediate aftermath of the Soviet implosion, when a significant amount of fissile material went missing, some of it undoubtedly stolen by former military officers and officials as a private pension plan. As in Africa, the diplomats are left with the challenging task of separating nuclear fact from fiction.

In July 2008, the embassy in Lisbon reported a "walk-in" informant with a tale of a retired Russian general who had a brick of uranium metal to sell. The informant handed over a picture of the merchandise – a lump of grey metal.

Noting that the "walk-in stated he is not on any medications and has not consulted any mental health specialists", the case is handed over to specialists, and there is no further mention of it in the cables.

More often, the post-Soviet nuclear black market has remained closer to home. An illicit trafficking database maintained by the IAEA records 500 incidents since the mid-90s, involving "the theft or loss of nuclear or other radioactive material". Of those, 15 involved high enrichment uranium (HEU) and plutonium, from which nuclear warheads are made. Most of those were in former Soviet republics or in eastern Europe.

As part of the "second line of defence" programme, the US National Nuclear Security Administration (NNSA) is helping Russia build radiation detectors on every one of the country's border crossings, about 350 sites, by the end of next year.

However, there are doubts that this expensive hi-tech approach will work. Critics point to the fact that out of 20 high profile cases where nuclear smuggling has been uncovered, radiation detectors have played a part in only one. That is partly because they are so easily triggered, there are often turned off or ignored by local officials.

A confidential cable from the US embassy in Tbilisi records an incident in August last year when a car carrying three Armenians set off a detector on the Georgian-Armenian border. The driver was waved on by customs guards, however, after he claimed to have been injected with radioactive isotopes during surgery.

The car was only searched when it set off the alarm again on the way back to Armenia. It was found to be contaminated throughout by Cesium-137, a highly radioactive isotope, which could make a devastating "dirty bomb". A cloth in the car produced the highest radiation reading, but no radioactive material was found. It appears that whatever the car was carrying, had been delivered.

Eight months later, two more Armenian smugglers crossed the same border carrying HEU, but managed to shield it from the detectors by the simple expedient of carrying it in a lead-lined cigarette box. The alarms did not sound, and instead, the two Armenians, Hrant Ohanyan and Sumbat Tonoyan, were caught in a Georgian sting operation and were sentenced last month to prison terms of 13 and 14 years respectively. As the US cables make vividly clear, their cases are the tip of an iceberg.