Russia has suspended talks with Turkey on a major gas pipeline project in the Black Sea, in a further escalation after Turkey downed a Russian warplane.
The TurkStream project was designed to boost Russian gas exports to Turkey - but now there are doubts about its future, as Russia tightens sanctions.
Turkey is the second biggest customer of Russian gas after Germany.
Russia began building part of TurkStream in May. It would have replaced the South Stream pipeline.
The suspension of talks was announced by Russian Energy Minister Alexander Novak.
Russia has announced wide-ranging trade sanctions against Turkey, in retaliation for the downing of an Su-24 bomber on the Syrian border on 24 November by Turkish F-16 jets.
Many Turkish food exports are affected, Turks will no longer enjoy visa-free travel to Russia, and Russians can no longer go on package holidays to Turkey.
Turkey's Prime Minister Ahmet Davutoglu has now said Ankara will speed up work on a pipeline called Tanap, to take delivery of Azerbaijani gas from the Caspian Sea.
Russia's state-controlled gas giant Gazprom says Russia and Turkey agreed in February on the TurkStream route. It was planned to come ashore at Kiyikoy, north-west of Istanbul, and continue up to the Greek border.
The annual capacity of TurkStream would be 63bn cubic metres (2,225bn cu ft).
Last year Gazprom exported 27.4bn cu m of gas to Turkey. It uses the Blue Stream pipeline, delivering gas to Turkey via the Black Sea.
Gazprom and Italy's energy group Eni built Blue Stream, which began operating in November 2005.
Russian President Vladimir Putin and Turkey's then Prime Minister Recep Tayyip Erdogan were at Blue Stream's launch, as close partners. Now they are trading bitter accusations over the downing of the Russian jet.
Russia cancelled the South Stream project a year ago. It would have delivered Russian gas to Eastern and Central Europe, but ran into opposition from the EU.
Russia's involvement in the conflict in eastern Ukraine has accelerated the EU's moves to diversify gas supplies, to make the bloc less reliant on Gazprom.