Syrian President Bashar al-Assad's government has condemned Turkey's military presence in northern Syria as a violation of a 1998 protocol between the two countries.

On Saturday, the foreign ministry in Damascus accused Ankara of repeatedly breaching the Adana deal throughout Syria's eight-year war.

"Since 2011, the Turkish regime has violated and continues to violate this agreement," a ministry source said, quoted by state news agency SANA.

The source accused Turkey of "supporting terrorists", using the government's term for both Turkish-backed rebels and other armed groups including former al-Qaeda affiliate Hay'et Tahrir al-Sham (HTS).

It said Ankara was breaching the deal through "occupying Syrian territory via terrorist organisations linked to it or directly via Turkish military forces".

But Turkey's President Recep Tayyip Erdogan has insisted the Adana Protocol gives his country the right to deploy troops in the neighbouring country.

Turkey backs Syrian rebel factions who control part of the north and has observation posts across rebel-held Idlib province as part of a "de-escalation" agreement signed in 2017 with Assad's ally Russia, and Iran.

Ankara has threatened for months to launch a new offensive in northern Syria to drive out US-backed Syrian Kurdish fighters.

It has twice led incursions into northern Syria in 2016 and 2018. Since then, its forces and allied Syrian proxies have controlled a patch of territory on the border.

Ankara has repeatedly threatened to march on areas further east, where the US-allied fighters of the Kurdish People's Protection Units (YPG) are in control.

The YPG is seen as an effective ground force by the US in the fight against the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) armed group, but Turkey says it is linked to the outlawed Kurdistan Workers' Party (PKK), which Ankara and Washington list as a terrorist group.

Washington last month said it would pull its 2,000 troops from the country, leaving the Kurds scrambling to find a new ally in Damascus to avoid a potential Turkish assault.

On Friday, Erdogan said Turkey expected a "security zone" to be created in Syria in a few months.

The Adana deal was signed in 1998 to end a crisis between the neighbours, sparked by the presence in Syria of PKK leader Abdullah Ocalan and bases run by the group.

Turkey argues the protocol provides Ankara with the legal ground to intervene in Syria against the PKK and its affiliates, because of the Syrian regime's failure to act against the group.

Damascus has regained control of almost two-thirds of the country after Russia intervened militarily in 2015.