Sunni insurgents have seized nuclear material used for scientific research at Mosul University, Iraq's UN Ambassador Mohamed Ali Alhakim has said in a letter to UN Secretary-General Ban Ki-moon.

In the letter, obtained by Reuters, Alhakim has requested for help to "stave off the threat of their use by terrorists in Iraq or abroad."

Alhakim said Mosul University was housing almost 88 pounds of uranium for scientific research, adding that it "can be used in manufacturing weapons of mass destruction."

"Terrorist groups have seized control of nuclear material at the sites that came out of the control of the state."

"These nuclear materials, despite the limited amounts mentioned, can enable terrorist groups, with the availability of the required expertise, to use it separate or in combination with other materials in its terrorist acts," said Alhakim.

Fears that Iraq was in possession of weapons of mass destruction helped spark the US invasion of Iraq in 2003. Those tales proved to be unfounded. On Wednesday, US officials told Reuters they do not believe the substance reportedly seized by insurgents is weapons-grade. In order for the uranium to be used as a weapon, it needs to enriched, a heavily complicated process.

While Alhakim did not specify which group seized the stockpiles, the city of Mosul was taken over by a Sunni extremist group, Islamic State of Iraq and Syria (ISIS) in June. Since then they have taken over several other cities in the country.

"The Republic of Iraq is notifying the international community of these dangerous developments and asking for help and the needed support to stave off the threat of their use by terrorists in Iraq or abroad," Alhakim wrote.

On Tuesday, a letter made public at the United Nations claimed that ISIS captured a facility near Baghdad stocked with sarin-filled rockets left over from the reign of Saddam Hussein.

The US, however, did not see it as a threat. "Whatever material was kept there is pretty old and not likely to be able to be accessed or used against anyone right now," US Defence Department spokesman Rear Admiral John Kirby told the New York Daily News.