NATO countries are meeting Tuesday in the Spanish capital Madrid, where the alliance is to adapt its military stance towards Russia as the war in Ukraine enters its fifth month.
Speaking ahead of the summit, Secretary-General Jens Stoltenberg said NATO would massively boost the number of troops on high readiness from 40,000 to over 300,000 - its "biggest overhaul" since the Cold War.
"Russia has walked away from the partnership and the dialogue that NATO has tried to establish with Russia for many years," Stoltenberg told reporters in Brussels on Monday.
"They have chosen confrontation instead of dialogue. We regret that - but of course, then we need to respond to that reality."
The two-day event is intended to chart a course "for a new security reality", which will involve building up NATO battlegroups in eastern Europe to "brigade levels", Stoltenberg added.
NATO is also expected to consider bids by Finland and Sweden to join the alliance despite opposition from Turkey.
At the summit Nato is expected to change its language on Russia from existing wording that describes Moscow as a strategic partner.
"I expect the allies will state clearly that Russia poses a direct threat to our security, to our values, to the rules-based international order," Stoltenberg said.
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On Sunday thousands of people took to the streets of Madrid to denounce the summit. The protesters say increased defence spending in Europe is a threat to peace.
Meanwhile Spain has been lobbying for NATO to pay more attention to security threats on its southern flank with Africa - a main gateway into Europe for irregular migration.
Madrid is equally concerned about lawlessness and violent Islamist movements in the Sahel region.
"We have this war in Europe, but the situation in Africa is really worrying," said Spanish Defence Minister Margarita Robles.