New sanctions against the Russian energy sector entered into force Friday in response to continued aggression in Ukraine, the European Union declared.

Herman Van Rompuy, president of the European Council, said the series of restrictive measures adopted Monday are now in force, barring Russian oil company Rosneft and its counterparts Transneft and Gazprom Neft from working in European capital markets.

"In addition, certain services necessary for deep water oil exploration and production, arctic oil exploration or production and shale oil projects in Russia may no more be supplied, for instance drilling, well testing or logging services," the European Council's package from Monday read.

The European Council said it opted to strengthen existing sanctions "in view of the gravity of the situation on the ground in eastern Ukraine."

Russia relies heavily on oil and natural gas revenues to support its economy. When earlier sanctions went into force, Andrei Belousov, an economic adviser to the Kremlin, said the Central Bank of Russia may sell some of its foreign currencies to blacklisted companies in an effort to offset the punitive measures.

A cease-fire agreement in eastern Ukraine was reached during multilateral talks in Minsk. The White House, which said it would mirror the latest European efforts, said the truce came as a result of the economic pressure on Russia.

For Van Rompuy, the measures may be scaled back should the situation change for the better.

"We have always stressed the reversibility and scalability of our restrictive measures," he said.