When it comes to nuclear material falling into the wrong hands, Pakistan, the former Soviet Union and North Korea top the list of countries posing the greatest risk, experts say.

But dozens of civilian research reactors around the world also offer an inviting target for extremists seeking to acquire the highly-enriched uranium or separated plutonium needed for an atomic bomb.

As President Barack Obama on Monday warned of the danger of "nuclear terrorism" at a 47-nation summit in the US capital, analysts said a volatile climate in Pakistan presented the most serious potential threat.

Although Pakistan's sensitive nuclear sites are under heavy guard, governments fear Islamist militants could steal weapons or fissile material with the help of allies inside the intelligence service.

The country's stockpile is a fraction of Russia's or some other nuclear powers, but the growing strength of extremists in Pakistan and tensions with nuclear-armed rival India offer a volatile mixture, analysts said.

"It's a much smaller program but we know that parts of the government are very sympathetic to Al-Qaeda," Daniel Byman, director Georgetown University's Center for Peace and Security Studies, told AFP.

Anti-India militants in Lashkar-e-Taiba, suspected in the 2008 siege of Mumbai that left 166 dead, represent the gravest danger to safeguarding Pakistan's arsenal, said Bruce Riedel, a former CIA officer.

Pakistan has yet to move against Lashkar-e-Taiba, amid speculation Islamabad's powerful spy service views the group as leverage against arch-foe India.

"If there is a nightmare nuclear security scenario in Pakistan today it is probably an inside the family job that ends up in a nuclear Armageddon in India," Riedel said on the Brookings Institution website.

In Russia and former Soviet republics, authorities have dramatically improved security at numerous nuclear sites that had lapsed amid chaotic conditions in the 1990s.

But an array of bunkers, labs and reactors across the former Soviet Union still lack adequate security and have yet to fully document the weapons-related material in hand.

"We know material is not accounted for. It could be just laying there in a warehouse, but we know there are real problems," Byman said.

North Korea has been accused of selling a reactor capable of producing plutonium to Syria, and Western governments worry the regime could be hawking its nuclear hardware and blueprints on the black market.

The suspected sale to Syria "suggests that they might be wanting to sell it to someone else," Byman said.

Governments have only recently woken up to the less publicized risk of stores of weapons-grade uranium at research reactors and other civilian facilities around the world, said a report issued Monday.

About 60 metric tons of highly-enriched uranium (HEU) is under civilian use or in storage, mainly at research reactors, with roughly half of it outside the United States or Russia, said the report, "Securing the Bomb," by a Harvard University analyst.

Despite some tighter security measures, the report said "it remains the case that most civilian research reactors have very modest security -- in many cases, no more than a night watchman and a chain-link fence -- even when enough fresh or irradiated HEU for a bomb is present."

In 2007, armed men broke into South Africa's Pelindaba research nuclear facility, where enough uranium for 30 nuclear weapons was stored. The uranium was left untouched but the gunmen escaped.

International efforts to safeguard materials have focused in part on securing uranium that was distributed by nuclear powers over several decades, under a program dubbed in the 1950s as "Atoms for Peace."

About 20,000 kilograms (44,000 pounds) of weapons-grade uranium was handed out, in exchange for UN inspections to ensure the material would be used only for peaceful purposes.

The US government has tried to secure the uranium and put it under safekeeping in recent years, but some countries have resisted, saying they need it to produce medical isotopes.