WASHINGTON — As Ukrainian leaders warned on Monday of “a great war” with Russia, NATO leaders meeting in Wales this week were expected to endorse their most concrete response yet to increased Russian military intervention in Ukraine: establishing a rapid-reaction force capable of deploying quickly to Eastern Europe, officials of the alliance said.

The new force of some 4,000 troops, capable of moving on 48 hours’ notice, will be supported with logistics and equipment pre-positioned in Eastern European countries closer to Russia, with an upgraded schedule of military exercises and deployments that are intended to make NATO’s commitment of collective defense more credible and enhance its deterrence.

The agreement is planned as the substantive centerpiece of the NATO meeting, which will take place Thursday and Friday and will be attended by President Obama, who will also stop in Estonia before the summit meeting. His aides said the trip was intended to highlight the United States’ commitment to NATO, and the alliance’s determination to protect all 28 members from aggression — from Moscow or elsewhere.

“The summit is very important because Russia thought it can change the borders of a sovereign European country by force, and this is happening not very far from NATO’s borders,” said one of Mr. Obama’s proud Estonian hosts, Prime Minister Taavi Roivas. “The security situation has changed, and we need to rethink our plans and reinforce our allies, so we can be 101 percent sure that all member states are equally and strongly protected.”