Georgian security forces have foiled a criminal plot to sell weapons-grade uranium on the black market, the country's president told a gathering of world leaders yesterday.

The revelation brought a sense of urgency to the Washington summit on nuclear security, where Barack Obama called on the rest of the world "not simply to talk, but to act" to destroy vulnerable stockpiles of nuclear material, or to safeguard them against theft by terrorists.

Last night, the US president welcomed an announcement by his Russian counterpart to shut down his country's last plant producing plutonium, in the Siberian, formerly closed city of Zheleznogorsk. Obama said it demonstrated "Russia's leadership on nuclear issues".

As part of its contribution to the cause of nuclear security, France offered to open a nuclear site to UN inspection and President Nicolas Sarkozy proposed that any leader found transferring nuclear material to terrorists should face trial before an international court.

Pakistan came under particular scrutiny in Washington because of widespread fears that extremists could steal a weapon from its nuclear arsenal, and because of its opposition to a treaty that would ban the production of fissile material.

But the revelation from Georgian president Mikheil Saakashvili of a nuclear smuggling case served as a reminder that there is still considerable uncertainty about whether the weapons and fissile material left behind when the Soviet Union collapsed have been fully accounted for.

Georgian sources said the highly-enriched uranium – HEU – was intercepted in a sting operation carried out by the Tbilisi authorities in March without international assistance. They said the uranium was more than 70% enriched. The exact analysis is expected in a few days, but it appears to have been pure enough to use in a crude nuclear weapon.

The amount seized was small, measured in grams, so nowhere near the 25kg minimum needed for a functioning bomb, but Georgian officials said the gang was offering the HEU as a sample of a bigger quantity available for purchase. The officials would not comment on the nationality of the gang.

"The Georgian ministry of interior has foiled eight attempts of illicit trafficking of enriched uranium during the last 10 years, including several cases of weapons-grade enrichment. Criminals associated with these attempts have been detained," the Georgian president told the summit. "The most recent case of illicit trafficking was the attempted sale of highly enriched uranium in March of this year."

The seizure echoes the case of Oleg Khinsagov, a North Ossetian smuggler who was arrested in 2006 trying to sell 100g of HEU in Georgia. He also claimed to his would-be buyers that the sample, which was found to be nearly 90% enriched, was just a sample from a bigger batch. Khinsagov is now serving an eight-year sentence, but this latest case suggests a bigger batch of stolen HEU may well exist.Since 1993, the International Atomic Energy Agency has confirmed 15 cases of smuggling of weapons-grade HEU or plutonium, but UN investigators admit they have no idea how much of the trade they are seeing. There are fears that only petty operators are being caught and that more organised gangs are evading capture.

Matthew Bunn, a leading expert on nuclear security at Harvard University, said: "One of the troubling things about past cases in Georgia is that there was some evidence that this was the activity of an organised criminal group and they were selling lots from the same batch of material. Its an interesting question whether this is the same group."

Bunn added: "I hope this puts the fear of God into other leaders and convince them that this a real problem and an ongoing problem."US officials pointed to some concrete successes from the summit. Ukraine and Canada said they would no longer use HEU in research reactors, and switch to low enriched uranium (LEU) – which is much harder to turn into a bomb. Canada's HEU will be shipped to the US, from where it originally came, for safekeeping. The Ukrainian HEU originally came from the Soviet Union and will be returned to Russia.. Malaysia announced it had enforced tougher controls on the shipment of nuclear equipment.

The summit affirmed the support of the 47 nations for Obama's goal of securing the stockpiles of fissile material within four years, and called for more countries to switch from HEU to low-enriched uranium reactors. It also called on the ratification of UN conventions aimed at setting international standards for nuclear security. Some countries, including France, pledged to accelerate the ratification process. There were pledges of more financial support for the International Atomic Energy Agency, which will play a newly enhanced role in monitoring and reinforcing global security standards.

There were no binding commitments in the communiqué or a more detailed workplan that was issued alongside it, but rather a political pledge to deliver results in the two years before the next summit, to be held in South Korea. Meanwhile, agreement was reached that top officials, or sherpas, from the 47 countries would meet every six months.