Milos Zeman said that there should be consequences if Russia decides to follow its annexation of the Black Sea peninsula of Crimea last month with further military action.
"If Russia decides to extend its territorial expansion to eastern Ukraine, the fun is over," he told public Czech Radio yesterday.
"In that case, I would promote not only the toughest EU sanctions possible, but also let's say military readiness on the part of NATO, for instance with its troops entering the Ukrainian territory," he added.
Crimea's largely Russian-speaking residents voted in March to become part of Russia, in a hastily organised referendum held as Russian troops patrolled the region.
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Several eastern regions with large Russian-speaking populations want to follow Crimea's example and stage referendums on joining Kremlin rule when Ukraine holds snap presidential polls on May 25.
Washington believes that Moscow has recently massed about 40,000 soldiers near the eastern border of Ukraine.
Although Moscow has denied plans to move its troops beyond Crimea, it has thus far pulled only a few hundred troops back from the border region.