Turkey's offensive in Syria: a timeline

Turkey's army launched a major air and ground operation in northern Syria on the weekend in a bid to oust a US-allied Kurdish militia that it considers a terror group.

The offensive follows an announcement by a US-led coalition fighting the Islamic State group that it is working to create a 30,000-strong border security force in northern Syria.

Around half of that force would be retrained fighters from the US-backed Syrian Democratic Forces, which is mainly made up of fighters from Kurdish militia the People's Protection Units (YPG).

Here is a timeline:

- Major incursion -

On Saturday Turkey launches operation "Olive Branch" into Syria intending to oust the YPG militia from its enclave of Afrin.

War planes and artillery back a major ground incursion undertaken with Ankara-backed Syrian rebels.

Turkey considers the YPG to be a "terrorist" group as well as the Syrian offshoot of the Kurdistan Workers' Party (PKK) which has waged a bloody three-decade insurgency against the Turkish state.

Ankara says it informed Syrian President Bashar al-Assad in writing of its offensive; this is denied by Damascus which says its air force will shoot down any Turkish warplanes entering its airspace.

Assad ally Russia urges restraint and withdraws troops from the zone in question "to prevent potential provocation".

The Kurdish militia say they will hold Russia as well as Turkey responsible for the attacks.

- 'Legitimate' concerns -

On Sunday Turkish tanks and soldiers enter the region of Afrin. Official media say that Ankara's forces have penetrated five kilometres (three miles) into Syria.

The US State Department calls on Turkey to "exercise restraint". But Defense Secretary Jim Mattis says Ankara has "legitimate" security concerns and had given Washington advance warning of the operation.

France calls for a UN Security Council meeting and calls on Turkey to end its offensive. Iran expresses concern and Syria's Assad condemns the operation.

Turkish police prevent protests against the operation in Istanbul and Diyarbakir, a Kurdish stronghold in Turkey's southeast.

- 'No step back' -

On Monday Turkish soldiers and their Syrian allies launch a new attack on the Kurdish militia from Azaz, about 20 kilometres to the east of Afrin.

"We will take no step back. We spoke about this with our Russian friends. We have an agreement," Turkish President Recep Tayyip Erdogan says.

Deputy Prime Minister Mehmet Simsek says the operation will be short. Ankara says it aims to create a security zone within 30 kilometres of the border.

The Kremlin says it is watching developments attentively and is in contact with Damascus and Ankara.

US Secretary of State Rex Tillerson urges restraint on all sides. But he also says Turkey has a "legitimate right to protect its own citizens from terrorist elements that may be launching attacks against Turkish citizens on Turkish soil from Syria".

The Kurdish forces say Turkey's operation amounts to supporting IS and urge their Western allies to act.

The Britain-based Syrian Observatory for Human Rights says 21 civilians -- including six children -- have been killed in the operation. Ankara says it has only hit "terrorists".