The Prime Minister has called on Nato members to send a "very clear, unified and strong message" in response to the crisis in Ukraine.
Speaking at a reception for the Nato Parliamentary Assembly at Downing Street, David Cameron said: "Members of Nato want and deserve real reassurance that our Article V commitments are rock solid."
Article V refers to one of the founding principles of the Nato alliance stating that if one of its members is attacked, it "shall be considered an attack against them all".
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.