The meeting of ambassadors is expected to be held on Wednesday, the source said, adding the entry into force of the package of sanctions approved by the EU Council on Monday was postponed for a few days, thanks first of all to Finland and Italy.
Finland's Prime Minister Alexander Stubb said on Monday that his country believed it was not suitable time to impose new sanctions and discussion on sanctions should continue.
Despite the fact that the three EU key institutions - the European Council, the European Commission and the EU External Action Service - said the EU Council should take a formal decision on the sanctions, they were not approved automatically.
A compromise decision was taken to declare that the EU may revise or suspend sanctions, depending on developments in Ukraine.
Finland and Italy, close business partners with Russia, oppose new European Union sanctions over events in Ukraine, the EurObservER internet portal said on Tuesday. And while Slovakia had not joined those critical of moves to penalize Russia, it was skeptical about anti-Russian action, said the online monitor of EU energy issues.
Its assessment follows new sanctions against Russia declared by the 28-nation bloc but delayed to judge how the Ukraine ceasefire is observed.
Brussels' announcement is the first reinforcement of sanctions approved since de-escalation of violence in Ukraine, Russian President Vladimir Putin's peace plan for southeast Ukraine and negotiations on a political settlement.
Media reported the new sanctions may target three largest Russian state-owned companies - Gazpromneft, Transneft and Rosneft to ban them from taking loans from EU countries for more than 30 days. The Uralvagonzavod and Oboronprom defence industrial companies and the United Shipbuilding Corporation may be also barred from EU loans.