Iraq has warned the UN that Sunni militants have seized nuclear materials used for scientific research at a university in the city of Mosul.
In a letter seen by Reuters, Iraq's envoy to the UN said nearly 40kg (88lb) of uranium compounds were seized.
The letter appealed for international help to "stave off the threat of their use by terrorists in Iraq or abroad".
But the UN atomic agency IAEA said the material was "low grade" and did not pose a significant security risk.
US officials have also reportedly played down the threat, saying the materials were not believed to be enriched uranium.
They added that it would be difficult for the rebels to use the materials to make weapons.
"Terrorist groups have seized control of nuclear material at the sites that came out of the control of the state," Ambassador Mohamed Ali Alhakim said in the letter.
"These nuclear materials, despite the limited amounts mentioned, can enable terrorist groups, with the availability of the required expertise, to use it separately or in combination with other materials in its terrorist acts," he added.
However, International Atomic Energy Agency (IAEA) spokesperson Gill Tudor said on Thursday it "would not present a significant safety, security or nuclear proliferation risk".
But she said that "any loss of regulatory control over nuclear and other radioactive materials is a cause for concern".
Mosul, some 400km (250 miles) north-west of Baghdad, was last month seized by Isis-led (Islamic State in Iraq and the Levant) insurgents.
The letter comes a day after Iraqi officials confirmed that the rebels were in control of a disused chemical weapons factory.
Iraq said the Muthanna complex, north-west of the capital Baghdad, housed remnants of rockets filled with sarin and other deadly nerve agents.
The UN and US have said the munitions are degraded and the rebels will be unable to make usable chemical arms from them.
Meanwhile, tensions are building between the central Iraqi authorities and the autonomous Kurdistan regional government, one day after Iraqi Prime Minister Nouri al-Maliki said Isis militants were being harboured in the Kurdish city of Irbil.
A spokesman for Iraqi Kurdish leader Massoud Barzani said that Mr Maliki had become hysterical and urged him to step down.
He said: "You have destroyed the country and someone who has destroyed the country cannot save the country from crises."
Kurdish ministers have also said they would not attend cabinet meetings until further notice in protest at Mr Maliki's comments.
In recent weeks, Isis insurgents have seized huge swathes of north-western Iraq. Kurdish troops moved into areas abandoned by the Iraqi forces during the Isis onslaught, including the oil-rich region of Kirkuk.
The UN has said at least 2,417 Iraqis, including 1,531 civilians, were killed in "acts of violence and terrorism" in June.
More than a million people have fled their homes as a result of the fighting.