Image Herman Van Rompuy, president of the European Council, called sanctions a “warning.” Credit... Julien Warnand/European Pressphoto Agency

Dan Yergin, chairman of Cambridge Energy Research Associates, said the new energy measures underscored how much ties had deteriorated. “A year ago, Western collaboration with Russia’s energy sector was one of the bright spots in what had become a dour relationship,” he said. “No longer.”

The carefully orchestrated actions on both sides of the Atlantic were intended to demonstrate solidarity in the face of what American and European officials say has been a stark escalation by Russia in the insurgency in eastern Ukraine. Until now, European leaders had resisted the broader sorts of actions they agreed to on Tuesday, and their decision to pursue them reflected increasing alarm that Russia was not only helping separatists in Ukraine but directly involving itself in the fighting.

They are “meant as a strong warning,” Herman Van Rompuy, the president of the European Council, said in a statement on Tuesday that was joined by José Manuel Barroso, the president of the European Commission. “Destabilizing Ukraine, or any other Eastern European neighboring state, will bring heavy costs,” the statement said.

President Obama said Russia’s economy would continue to suffer until it reversed course. “Today is a reminder that the United States means what it says, and we will rally the international community in standing up for the rights and freedom of people around the world,” he told reporters on the South Lawn of the White House.