Sana'a official told US diplomats solo sentry had been removed from atomic facility and CCTV system was broken

A senior government official in Yemen warned US diplomats that poor security at the country's main store of radioactive products could allow dangerous material to fall into the hands of terrorists, according to a leaked US embassy cable.

The official told the Americans that the lone guard standing watch at Yemen's national atomic energy commission (NAEC) facility had been removed from his post and that its only closed circuit TV security camera had broken down six months previously and was never fixed.

"Very little now stands between the bad guys and Yemen's nuclear material," the official warned, in a cable dated 9 January this year sent from the Sana'a embassy to the CIA, the FBI and the department of homeland security as well as the US secretary of state in Washington and others.

Yemen, the Arab world's poorest nation, has emerged as al-Qaida's most active base, after Iraq and Afghanistan. It is home to Al-Qaida in the Arab Peninsula (AQAP), the group behind a series of attacks on western targets, including the failed airline cargo bomb plot in October and the attempt to bring down a US passenger jet over Detroit on Christmas Day last year. The Nigerian-born Detroit bomber, Umar Farouk Abdulmutallab, was radicalised in Yemen, according to security sources.

The cable, classified as secret by the US ambassador Stephen Seche, and sent during the immediate aftermath of the Christmas Day bomb, describes how the "worried" official implored the US to help convince the Yemen government "to remove all materials from the country until they can be better secured, or immediately improve security measures at the NAEC facility".

The cable revealed that the facility holds large quantities of radioactive material used by hospitals, local universities for agricultural research and in oilfields. The international community fears radioactive isotopes could be used to make a dirty bomb – a device combining simple explosives with radioactive materials, which it would disperse over a wide area.

The isotopes are not explosive themselves, unlike nuclear material such as uranium. Although unlikely to kill a large number of people, such a device could cause tremendous damage and disruption by creating large no-go areas contaminated by radioactivity.

International experts said today the lack of security at the Yemen facility would be a "high priority" for the US government. Told of the cable's revelation of the type of materials and the amount stored in Yemen's NAEC facility, Matthew Bunn, a former White House science adviser who specialises in nuclear threat and terrorism, said: "Holy cow. That's a big source.

"If dispersed by terrorists it could make a very nasty dirty bomb capable of contaminating a wide area," said Bunn, an associate professor at Harvard University's John F Kennedy school of government, who compiles an annual assessment of the nuclear terrorism threat titled Securing the Bomb.

Such a bomb would be "enough to make a mess that would cost tens of billions of dollars in cleanup costs and economic disruption, with all sorts of controversy over how clean is clean, how will people go back there", he said.

"It's the type of thing that the US programme have been working on securing all over the world. The global threat reduction initiative (GTRI) in the department of energy has two missions: one, to get rid of enriched uranium and two, to improve security on radioactive facilities so that dirty bombs cannot be used.

"The location in Yemen is obviously of particular concern given terrorism, given Al-Qaida in the Arab Peninsula headquartered there, also the spotty effectiveness of the government.

"I would think it would be a high priority to do something about it."

While a dirty bomb has never been detonated, terrorists have been accused of plotting such attacks.

A Briton, Dhiren Barot, admitted plotting to build a radioactive bomb in the UK and was convicted in 2006.

The leaked US cable revealed that, in the days following the official's warning over security and probably as a result of US diplomatic pressure, the radioactive material was moved to a more secure facility and the remainder of it was likely to follow.

In a section of the cable titled Comment, it read: "Post will continue to push senior ROYG (Republic of Yemen Government) officials to increase security at all national atomic energy commission facilities and provide us with a detailed accounting of all radioactive materials in the country."

A spokesman for the US state department said: "We decline to comment on any cable. A team from the US department of energy visited Yemen in February and continues to work with the government on security upgrades at relevant sites as part of its global threat reduction initiative."

The US national nuclear security administration declined to comment on the cable or any action taken as a result of it.

A spokesman added: "I am not going to comment on upgrades to any specific sites. I can say that we have programmes to co-operate with more than 100 countries around the world to secure vulnerable nuclear material, improve security at nuclear facilities, and prevent nuclear smuggling. We are working day and night to prevent terrorists from acquiring nuclear material, no matter the source."