At a separate news conference in Brussels on Wednesday, Maja Kocijancic, a spokeswoman for Catherine Ashton, the European Union’s foreign policy chief, said preparations were underway among the bloc’s leadership to strengthen sanctions against Russia if necessary.

Image Anders Fogh Rasmussen, the secretary general of the North Atlantic Treaty Organization, said on Wednesday that NATO would increase patrols along its eastern border in response to Russian interference in Ukraine. Credit... Laurent Dubrule/Reuters

“When it comes to Stage 3, the preparatory work is in an advanced stage,” said Ms. Kocijancic, referring to a possible toughening of measures that are already in place.

Ms. Kocijancic declined to comment on whether European Union divisiveness over how to react to evidence of growing Russian interference in Ukraine was affecting plans for a possible summit meeting next week. She said it would be up to Herman Van Rompuy, the president of the European Council, to decide whether to hold such a meeting.

The announcement that NATO was strengthening its presence in Eastern Europe heightened nerves about Ukraine across all of Europe.

While officials did not want to speak on the record, the announcement fed palpable doubts about whether four-way talks scheduled for Thursday in Geneva — bringing Russia and Ukraine to the same table for the first time since February, with the United States and European Union joining in — would proceed as planned.

Germany, which has Europe’s strongest economy and is the most important European trading partner for Russia, was at pains ahead of the scheduled meeting to stress that the Continent spoke with one voice on sanctions against Russia.

Chancellor Angela Merkel of Germany and other leaders here have for weeks made it clear that they will go along with tougher sanctions, while cautioning behind closed doors that a serious rupture in business and other ties could take years to heal.