(CNN) -

Militants in Iraq have taken hold of nuclear materials at university science facilities near the northern city of Mosul, the Iraqi government has said in a letter to the United Nations.

But two U.S. officials told CNN on Wednesday that the small amounts of uranium aren't enriched or weapons-grade, prompting only minimal concern.

The letter from Iraq's U.N. ambassador about the uranium compounds asks for help "to stave off the threat of their use by terrorists in Iraq or abroad" as the country struggles with a deadly insurgency.

In the letter, obtained Wednesday by CNN, Iraqi Ambassador Mohamed Ali Alhakim said that "terrorist groups have seized control" of nearly 40 kilograms (90 pounds) of uranium compounds at science departments at the University of Mosul after the sites "came out of control of the state."

The Islamic State in Iraq and Syria, an al Qaeda splinter group, has led Sunni insurgents who have taken over large areas of northern and western Iraq in an offensive that began last month. The terrorist group has also made major gains in Syria in its quest to establish an Islamic state spanning both countries.

In his letter, dated Tuesday, Alhakim said the nuclear materials were used in "very limited quantities" for scientific study and research. But he warned that despite the small amounts, the materials could be used by terrorists in Iraq or smuggled out of the country.

"Such materials can be used in manufacturing weapons of mass destruction," Alhakim wrote in the letter, which was first reported by Reuters.

Iraq witnessed another violent day Wednesday as the country's security and political crises deepened.

More than 50 unidentified bodies were found in the predominantly Shiite town of Alexandria on Wednesday, Iraqi security officials said.

The bodies of two children were among the dozens found in different parts of the town.

Details about the circumstances of the deaths were not immediately available, and officials did not say when the people may have been killed.

Not far from Alexandria, at least five people were killed and 17 wounded by three car bombs that exploded in front of a courthouse in the town of Hilla, security and medical officials said.

Hilla is about 92 kilometers (57 miles) south of Baghdad and is the first sizable town south of the capital.