In a statement on Wednesday, the G7 group of economic powers said Russia had undermined “Ukraine’s sovereignty, territorial integrity and independence.”
The warning came after the EU added eight more Russians to its sanctions.
Earlier, Russia described new US and EU sanctions as “destructive and short-sighted”, and said they would lead to higher energy prices in Europe.
The G7 group includes the US, Canada, France, Germany, Italy, Japan and Britain.
Its leaders said Russia could still “choose the path of de-escalation,” but warned President Vladimir Putin that he would face greater economic costs if he continued to back Ukrainian separatists.
They also called on all sides to establish a ceasefire at the crash site of the Malaysian Airlines jet that was shot down on July 17 in eastern Ukraine.
Russia has come under increased pressure to end its support for the rebels, who Western governments believe were behind the downing of MH17, killing all 298 people on board.
Vladimir Putin has also been accused by the US and EU of supplying heavy weapons to the rebels – a charge his government has denied.
On Tuesday, the US announced new economic sanctions against Russia, widening their scope to include three key sectors of the economy – energy, arms and finance.
The EU is also expanding its sanctions, targeting the oil sector, defense equipment and sensitive technologies.
Details of new EU sanctions are due to be published on Thursday.
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