ST PETERSBURG, Russia (REUTERS) - Russian President Vladimir Putin said on Friday that Moscow's biggest concern over the crisis in Ukraine was that the former Soviet republic would join Nato. "Tomorrow Ukraine may join Nato, while the day after tomorrow parts of the US anti-missile system could be deployed there," he told an investment forum.

Russia has long been wary of the expansion of the military bloc into eastern Europe, and in particular former Soviet republics. Putin said last month Russia's decision to annexe Crimea from Ukraine was partly influenced by concerns over Nato enlargement.