Rebel forces in Libya say they have taken control of the coastal city of Sirte, in what would be a key strategic gain.

Sirte has been controlled by forces aligned with Libya's UN-backed Government of National Accord (GNA) since Islamic State (IS) militants were expelled in 2016.

The rebel forces, loyal to Gen Khalifa Haftar, have been waging an offensive targeting the capital Tripoli.

The GNA has not yet commented.

A resident of the northern city of Sirte, located on the Mediterranean coast, told Reuters news agency they could see convoys of troops belonging to Gen Haftar's self-styled Libyan National Army (LNA).

"They control large parts of the city now. We also hear gunfire," the unnamed resident added.

The reports came after Turkish President Recep Tayyip Erdogan announced that his country's troops had begun moving into Libya to support the GNA in Tripoli.

The UN-backed government is supported by Turkey and its ally Qatar, while Gen Haftar is backed by Egypt and the UAE.

What is happening in Sirte?

The LNA said it had taken control of Sirte following a rapid advance on the city preceded by air strikes.

"Sirte has been totally liberated... from terrorist groups," Ahmad al-Mesmari, spokesman for Gen Haftar's forces, said on television.

It came after a series of posts on Facebook in which the group said it had taken control of key positions, including the local airport, and was making steady progress "towards the heart of Sirte to cleanse it from the grip of armed militias".

The GNA has not responded publicly to the reports of the takeover.

Sirte is located roughly midway between Libyan rival armed forces and authorities based in the east and west of the country. It is also near the resource-rich Oil Crescent region.

What's the background?

Libya has been torn by violence since long-time ruler Muammar Gaddafi was deposed and killed in 2011 by Nato-backed forces.

The country has two rival administrations, the UN-backed one based in Tripoli, and Gen Haftar's one in the eastern city of Tobruk.

The LNA launched its military offensive against the capital Tripoli last April. The group has intensified its efforts to gain ground in recent weeks as Turkey prepared to ramp up military support for the GNA.

Rebel forces were blamed for an air strike on a military academy on Saturday that killed at least 30 people. They denied any involvement.

The Turkish government has given no details about the scale of its military deployment.