Britain called on Thursday for Europe to hold firm over Ukraine's right to join NATO, as Prime Minister Boris Johnson visited the alliance headquarters to rally allies and sent his foreign secretary to Moscow with a warning not to invade.

Russia has more than 100,000 troops at the Ukrainian border, and the United States, Britain, the EU and NATO fear it may be planning an invasion. Moscow denies this but says it could take unspecified "military-technical" action unless demands are met, including barring Ukraine from ever joining the Western military alliance.

Britain's diplomatic push comes as...