Baghdad authorities have asked for international help after Sunni insurgents controlling parts of the country seized radioactive materials used for scientific research.

Led by the Islamic State (formerly known as ISIS), militants have stolen 40 kg of uranium from the university in Iraq's second-largest city Mosul, which fell into insurgent hands in early June as part of their offensive across the country and in neighboring Syria.

A letter to the United Nations quoted by the BBC calls on the international community to help "stave off the threat of [the seized uranium]'s use by terrorists in Iraq and abroad.

The UN's International Atomic Energy Agency (IAEA) sees no significant "safety, security or nuclear proliferation risk" posed by the rebels' action and believes the uranium was not enriched.

It however considers the "loss of regulatory control" over such materials to be "a cause of concern".

The Islamic State aims to expand what it calls a "Caliphate" in territories it has managed to seize in Iraq and Syria.

Over the past weeks it has taken over a number of towns and cities in both countries despite efforts by governments in Baghdad and Damascus to drive them off.

In the turmoil created by the advance and gains of the Sunni militants, Iraqi Kurdistan President Massoud Barzani has called on the region's parliament to draft a referendum on independence from Iraq.

Iraqi Prime Minister Nouri al-Maliki was quoted by Al-Jazeera as saying in response that the regional capital Erbil is becoming an operational base for the Sunni fighters.

Washington in other countries have been trying to press Maliki into creating a new government including representatives of three major groups in Iraq (Shia, Sunni, Kurdish representatives) to ease down tensions.