The source said the 18 entities will be made up of nine companies and nine institutions, such as the local authorities set up by pro-Russian rebels that have proclaimed independence in eastern Ukraine.
The ambassadors from the 28 member states were following through on a decision taken by EU leaders last week to extend the sanctions list.
There are currently 72 names hit with visa bans and asset freezes after Russia showed no sign of meeting demands to reverse course and cut support for the rebels.
The EU ambassadors were also discussing plans to move beyond the bloc's current mix of visa bans and asset freezes, under pressure to do more after the alleged shooting down of Malaysia Airlines flight MH17 last week by the rebels using a Russian-made missile.
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EU foreign ministers had agreed on Tuesday to speed up sanctions against Russia and to examine tougher measures, including in the defence sector, as a result.
They said the new list would accordingly include "entities and persons, including from the Russian Federation," for their role in stoking the crisis and aiding Russia's annexation of Crimea in March.
Such tougher measures, however, have proved very divisive because some EU member states, such as Germany and Italy, have extensive economic ties with Russia which they fear could be harmed by wider sanctions.
Other states, led by Britain and including former Soviet-era states such as Poland and Lithuania, want much tougher action.
Washington too has pressed for a harder line, announcing last week that it planned to curb Russian access to its financial markets.