Turkish President Recep Tayyip Erdogan and Russian counterpart Vladimir Putin on Monday marked the completion of the offshore phase of a gas pipeline underneath the Black Sea, the latest sign of burgeoning co-operation between Ankara and Moscow.

Erdogan hailed the TurkStream pipeline, which aims to pump some 31.1bn cu m of gas from Russia to Turkey annually, as a "new step" in Turkish-Russian energy cooperation, which he said showed the "high level" of relations between the two countries.

Yesterday's ceremony marked the completion of the building of two undersea lines stretching 930km (578 miles) across the Black Sea from Anapa in Russia to Kiyikoy in Turkey at a depth of some 2km (6,500ft).

The pipeline was laid by the special pipe-laying vessel Pioneering Spirit, which is the area of some six football pitches.

Putin and Erdogan watched via video link in Istanbul as the last section was welded and laid into the sea by the vessel.

Putin said he believed TurkStream and the Akkuyu nuclear power station would become "clear symbols of the growing development of Russia and Turkey's multi-faceted partnership."

"This (TurkStream) will without doubt turn Turkey into a serious European hub and this will without doubt have an effect on the geopolitical position of the Turkish Republic," he added.

The onshore section of the pipeline in Turkey still needs to be built and TurkStream expects the gas to start being pumped at the end of 2019.

The aim is that half of the gas pumped through the pipeline will go to ensure the energy needs of western Turkish cities like Istanbul, Bursa and Izmir and the other half sent on to other European countries.

Despite in theory being on opposite sides of the Syrian civil war, regime backer Russia and rebel supporter Turkey have worked closely to end the conflict and stave off a government assault to re-take the key region of Idlib.

Meanwhile, Ankara and Moscow have agreed terms on the delivery of S-400 missiles in a deal that had alarmed Nato member Turkey's Western allies.

"We have never determined our bilateral relations with Russia according to demands or pressure from other countries," Erdogan said to loud applause.

Putin meanwhile personally praised Erdogan, saying the TurkStream project could not have been released without the Turkish president.

"Such a project needs political will and courage. Because in the circumstances of growing competition such projects cannot be without this," he said.

Both leaders also reaffirmed their goal to lift annual bilateral trade volumes to $100bn, which Putin said was the same as between Russia and China.

"Why should it be less with Turkey? We will achieve this result. I don't even doubt this," said Putin.