NATO foreign ministers have unanimously agreed a set of measures in response to Russia's annexation of Crimea, including the formal suspension of all practical civilian and military cooperation with Moscow.
Ministers also agreed to reassess current military deployments among NATO's eastern European members.
Speaking after meeting in Brussels, NATO's secretary general, Anders Fogh Rasmussen, described Russia's annexation of Crimea as the gravest threat to European Security for a generation.
Earlier he rejected Russian claims that it was pulling some forces back from its border with Ukraine. NATO estimates there are now about 50,000 Russian troops on the Ukrainian border and in Crimea.
Rasmussen said Russia's actions were unacceptable and warned that as a result "Russia has undermined the principles on which our partnership is built, and has breached its own international commitments. So we cannot go on doing business as usual."
Contacts between NATO and Russia at ambassadorial level and higher will continue, so they can discuss ways out of the crisis.
According to one NATO official, the ministers also ordered military planners to "develop as a matter of urgency a series of additional measures to reinforce NATO's collective defenses."
The official, quoted by Reuters news agency, said the measures could include:
- Sending NATO soldiers and equipment to NATO allies in eastern Europe;
- Holding more exercises;
- Taking steps to ensure NATO's rapid reaction force could deploy more quickly;
- A review of NATO's military plans.
Meanwhile, at a separate meeting, foreign ministers from NATO and Ukraine agreed to step up cooperation but did not give details. This could include sending NATO military personnel to Ukraine for training purposes.
"It was a good decision," says former US ambassador to NATO, Nicholas Burns, speaking about the decision to end cooperation with Russia. Burns says Russia has to pay a price for what he calls its "blatant violation of international law."
"NATO began its relationship with Russia in a serious way back in 2002, just after 9/11," Burns says. "And the idea was that we'd work together for peace and security in Europe, to secure borders, not to violate them."