The unofficial government controlling Tripoli and parts of western Libya has sent troops to Sirte to secure state institutions, its military said. REUTERS

Islamic State (Isis) militants have seized the university in the central Libyan city of Sirte, according to residents.

A picture posted on social media showed the Islamic State flag at the university's entrance.

The group is said to have taken control of the city's administrative complex and expelled all staff.

Meanwhile the unofficial government, operating from Tripoli, accused Gaddafi loyalists in Sirte of using the name of Islamic State to undermine Libya's reputation.

The government has sent troops to Sirte to secure state institutions, a military source said.

Its head, Omar al-Hassi, accused Egyptian President Abdel Fattah al-Sisi of coveting Libya for its oil and gas.

Reports from Sirte residents who refused to be named said the militants had also taken over a radio station and other government buildings.

A video on social media, released two days ago, showed a convoy of vehicles flying Islamic State flags driving through the city.

Sirte, the birthplace of former Libyan leader Muammar Gaddafi, has been a stronghold of Ansar al-Sharia, the Islamist group suspected of carrying out the Benghazi attack on the US embassy in 2012 which killed ambassador Chris Stevens.

Islamic militants of various groups have taken advantage of the volatile ground situation in the absence of a strong government.

Libya's internationally recognised government and elected parliament has been confined to the country's east after a rival Islamist group called Libya Dawn seized the capital Tripoli in August, reinstating the previous assembly and rival government.

Libya's three main cities of Tripoli, Benghazi and Misrata are largely controlled by militias opposed to the UN-backed government based in the eastern city of Tobruk.

Factional fighting has left Libya in chaos for almost four years after the fall of Gaddafi. Two competing governments are scrambling for control and the Islamist militants have made the most of the situation in the oil-rich North African nation.

More recently, the Islamic State released a video showing execution of 21 Egyptian Christians. Cairo was quick to retaliate with air strikes on the militant camps in Libya.

Egypt and the official Libyan government have asked the United Nations Security Council to lift an arms embargo and help the army tackle the jihadists.