BRUSSELS, Sept. 1  -- NATO Secretary General Anders Fogh Rasmussen on Monday said the U.S.-led alliance will plan to build a multinational force to make NATO more agile and to respond to Russia's "aggressive behavior."

"We will now significantly enhance the responsiveness of our NATO Response Force. We will develop what I would call a spearhead within our Response Force -- a very high readiness force able to deploy at very short notice," Rasmussen told a press conference in Brussels.

He added NATO will agree a readiness action plan to make NATO more agile than ever so as to respond to Russia's "aggressive behavior."

"This is a multinational force, which brings together land, air, maritime and special operation forces. It can be deployed anywhere in the world, for collective defense or crisis management," he said.

"This spearhead would be provided by Allies in rotation, and could include several thousand troops, ready to respond where needed with air, sea and Special Forces support," Rasmussen added.

NATO will hold a summit meeting in Wales on Thursday, expected to focus on how to increase defense investments amid the escalating Ukraine crisis. Ukrainian President Petro Poroshenko will also attend the meeting.

The NATO chief said the Alliance's leaders will make clear their support for Ukraine as it is confronted by "Russian aggression." Leaders are expected to discuss Poroshenko's reform priorities so as to take concrete steps to help Ukraine.

Meanwhile earlier in the day, Russian Foreign Minister Sergei Lavrov said that Moscow will not militarily interfere in the ongoing Ukraine crisis, urging the United States and the European Union to help stop Kiev from using heavy weapons.

Lavrov expressed the hope that the Contact Group for Ukraine, which are meeting in the Belarussian capital of Minsk on Monday, primarily dedicate to "an imminent and unconditional ceasefire."

In mid-April, Ukraine's military launched offensives against insurgents to regain control over the seized cities and towns in eastern part of the country.

The armed conflict has claimed at least 2,500 lives and displaced thousands of people.

The Wales summit will also focus on NATO's tasks in Afghanistan.

"We will prepare a new chapter in our relationship with Afghanistan, as our combat mission draws to a close," Rasmussen said.

"We have planned a new mission to train, advice and assist the Afghan forces from next year and it will be launched once we have the legal arrangements in place," he added.