NATO 'on alert' as Russia moves troops to Belarus

"We are working hard for the best peaceful, political solution but we're also prepared for the worst," he said.

The former Norwegian prime minister said the U.S.-led military alliance was currently providing support to Ukraine and stepping its presence in Eastern Europe.

"We're also ready to respond if Russia chooses an armed conflict confrontation," Stoltenberg said, adding it was not planning to deploy combat-ready troops to Ukraine, which is not a member of NATO.

The comments came as President Vladimir Putin said the United States and NATO had not addressed Russia's main security demands in their standoff over Ukraine but that Moscow was ready to keep talking.