"But we don't want to change borders," he added.
We do have a quarrel with Putin, or Russia, trying to change borders by force".
The problem is that none of these solutions will satisfy everyone, and any redrawing of existing borders could set off a move to change borders elsewhere in the Balkans.
He said: It is completely unacceptable for Russia to use force to change borders, on the basis of a sham referendum held at the barrel of a Russian gun.
"Nations must not attempt to change borders or zones of occupation through armed force," she said, adding that "tangible steps" should be taken to respect the Line of Control.
The idea of collective defence has become more important given how Russia is using force to change borders in Europe".
"There is no place for the use of force and coercion to change borders on the European continent in the 21st Century".
At a defence conference on Thursday, Nato chief Jens Stoltenberg said that a more assertive Russia had been using force to change borders and intimidate its neighbours.
UK Prime Minister David Cameron said: "It is completely unacceptable for Russia to use force to change borders on the basis of a sham referendum held at the barrel of a Russian gun".
"To the east, we see a resurgent Russia, willing to change borders by force.
