Moscow and Ankara 'have almost finished discussing' the technical side of the contract for S-400 air-defence system.

Russia is closing in on a deal with Turkey to supply its latest S-400 air-defence system to Ankara, Moscow said on Saturday, in the latest sign of restored ties.

"Moscow and Ankara have almost finished discussing the technical side of the contract for S-400," Russian state giant Rostec said in a statement.

The firm said the finance ministries from the two countries were "discussing the possibility of providing Ankara a loan for the purchase".

Rostec CEO Sergei Chemezov said he did not know the outcome of the financial negotiations and refused to give any more details on the sale as the "contract is not yet signed".

The potential sale of the hi-tech system by Russia to NATO-member Turkey marks the latest step in a dramatic turnaround in relations since the two fell out after Ankara downed a Russian warplane on the Syrian border in 2015.

Leaders Vladimir Putin and Recep Tayyip Erdogan eventually managed to patch up ties and Russia has since rolled back most of the retaliatory sanctions it slapped on Turkey, except for a ban on tomatoes and scrapping of visa-free travel.

Russia moved the S-400 to Syria after Turkey shot down its warplane to protect its jets flying a bombing campaign in support of President Bashar al-Assad.

Ankara and Moscow are on opposing sides of the Syrian conflict with Moscow backing Assad and Turkey arming rebels, but have made a joint push to end fighting since late last year.