The Nord Stream 1 pipeline restarted following a 10-day maintenance break but at a reduced level.
On Wednesday, the European Commission urged countries to cut gas use by 15% over the next seven months in case Russia switched off Europe's supply.
Russia supplied Europe with 40% of its natural gas last year.
Germany was the continent's largest importer in 2020, but has reduced its dependence on Russian gas from 55% to 35%. Eventually, it wants to stop using gas from Russia altogether.
Russia's invasion of Ukraine in February sent wholesale gas prices soaring in Europe, with a knock-on impact on consumer energy bills.
The European Commission says retail electricity prices for households in EU capitals were up by 44% in May 2022, compared to May 2021. The highest rises were in the Netherlands (up 167%), Austria (up 122%) and Italy (up 118%).
Russia's President Vladimir Putin has sought to play down fears, promising that state gas firm Gazprom would fulfil all its contractual obligations. His spokesman, Dmitry Peskov, denied that Russia was using gas for political blackmail.
But the pipeline is only delivering 40% of its capacity, and the head of Germany's network regulator warned that the resumption of gas flows was not a sign that tensions were easing.
"The political uncertainty and the 60% cut from mid-June unfortunately remain," Klaus Müller said on Twitter.
Gazprom cut the flow of gas through Nord Stream 1 last month, blaming the delayed return - due to sanctions - of a key piece of equipment which had been serviced in Canada.
The turbine is now believed to be on its way back to Russia, but President Putin recently said that if it is not returned, supply would have to be reduced further.
He has also pointed out that another machine is due to be sent for service soon, according to Russia's Tass news agency.