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Islamic State militants in Libya say they have seized the airport in the city of Sirte, as the group continues to make advances in the country.

The news was announced by the group and by a Libyan militia that withdrew from the coastal city's airport on Thursday.

Most of Sirte, former leader Muammar Gaddafi's hometown, fell to IS last week.

In a statement, the group said it had also seized the Great Man Made River water project.

The irrigation project, the world's largest, supplies fresh water to Libyan cities and was also the base for the opposition Battalion 166, which has now fled.

The battalion, and other Islamist militias, run the capital, Tripoli, although their government is not recognised by the international community.

The al-Ghardabiya airport also housed an airbase, that was seized by militias after being bombed by Nato forces during the battle to oust Gaddafi in 2011.

Since the former leader's death in Sirte in October that year, Libya has descended into chaos, with various militias fighting for power. The internationally-recognised government has been forced to leave Tripoli.

In the last seven months, IS has set up checkpoints and established a presence in cities across Libya, starting with the port of Derna - the first city outside Iraq and Syria to fall to the militant group.

In January, an IS-affiliated group bombed the Corinthia Hotel in Tripoli, killing 10 people.

In February and April, videos were released showing IS militants in Libya killing two groups of mainly Egyptian and Ethiopian Christians.

Air strikes against IS positions have been conducted by the Egyptian air force and by Libya's internationally-recognised government.