Russian President Vladimir Putin told NATO countries on Thursday they risked nuclear conflict if they sent troops to Ukraine, adding that Russia must strengthen its western military district after Finland and Sweden's admission to the Atlantic alliance.

The United States and key European allies this week said they had no plans to send ground troops to Ukraine, after France hinted at the possibility.

Putin issued the warning during his annual address to Russia's parliament and other members of the country's elite. (Reporting by Vladimir Soldatkin; Writing by Alexander Marrow Editing by Gareth Jones)