The head of NATO pledged on Wednesday to immediately step up military patrols along the alliance’s vast eastern border in response to mounting evidence of Russian interference in Ukraine. The announcement by Anders Fogh Rasmussen, the secretary general of the North Atlantic Treaty Organization, appeared to be another sign that the confrontation with Russia over Ukraine was becoming more acute. But Mr. Rasmussen emphasized that the move was designed to be a deterrent rather than preparation for conflict. “Our decisions today are about defense, deterrence and de-escalation,” Mr. Rasmussen said in a statement posted on the NATO website. “NATO will protect every ally and defend against any threat against our fundamental security,” he said. Even so, the moves represent a significant strengthening of NATO’s posture in a region where it is already operating an air-policing mission in the Baltic states and surveillance flights over Poland and Romania. Mr. Rasmussen said the decision on Wednesday meant that aircraft will fly more sorties over the Baltic region and that allied ships will deploy to the Baltic Sea, the eastern Mediterranean and elsewhere, as required.  In addition, military personnel will deploy “to enhance our preparedness, training and exercises,” he said. “We will have more planes in the air, more ships on the water, and more readiness on the land,” he said.

The measures will be implemented “straight away,” he said, and “more will follow, if needed, in the weeks and months to come.” Mr. Rasmussen also renewed calls on Russia to stop destabilizing Ukraine, to pull back its troops from the Ukrainian border and to “make clear it doesn’t support the violent actions of well-armed militias of pro-Russian separatists.” 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. “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. More