Russian President Vladimir Putin has said foreign states should understand: "It's best not to mess with us."
The premier said his armed forces were ready to meet any aggression and that the Ukraine crisis was the result of a refusal by Kiev to negotiate.
He added: "Thank God, I think no one is thinking of unleashing a large-scale conflict with Russia. I want to remind you that Russia is one of the leading nuclear powers."
Kiev has said it wants supplies of weapons from Nato but does not expect them to send soldiers in to help fight Russian troops.
Were the Russians -as Barack Obama claims - weighed down by sanctions, or where the Ukrainians looking to cut their losses?
The new president of the European Council could be "very important" for David Cameron.