Rosneft is one of the companies targeted by the new sanctions
European Union member states have formally adopted new sanctions on Russia over the Ukraine crisis.
The new measures include restrictions on large Russian state-owned oil companies raising money on European financial markets.
However, the new measures will come into effect "in the next few days", not on Tuesday as some had expected.
Russia denies accusations by Ukraine and the West that it has been sending troops to help pro-Russian rebels.
EU Council President Herman van Rompuy said the measures were aimed at "promoting a change of course in Russia's actions destabilising eastern Ukraine".
But the 28-member bloc is being deliberately vague about when they will come into force, to allow time to assess the implementation of a ceasefire agreed on Friday, BBC Europe correspondent Chris Morris reports.
"Depending on the situation on the ground, the EU stands ready to review the agreed sanctions in whole or in part," Mr van Rompuy said.
The ceasefire appears to be holding, although the head of the Organization for Security and Cooperation in Europe (OSCE), which brokered the deal, described it on Monday as "shaky".
The gas sector is not affected by the latest sanctions. However, major state-owned oil firms are included, such as Rosneft, which is already targeted by US measures.
Russia has warned that it could block international flights through its airspace if the EU goes ahead with new measures.
Diplomats say the new package will target Russian oil companies Rosneft and Transneft and the petroleum unit of state gas monopoly Gazprom.
Their access to financial markets will be restricted - a serious matter for Rosneft, which last month asked the Russian government for a $42bn (£25.2bn) loan.
