U.S. warns of further sanctions on Russia if Crimea formally annexed

WASHINGTON (Reuters) - Russian could face additional sanctions if it proceeds with formal annexation of Ukraine's Crimea region or takes further military action in Ukraine, a senior U.S. official said on Monday.

There is "concrete evidence" that some ballots arrived "pre-marked" in Sunday's secession referendum in Crimea and other voting anomalies were also detected, another U.S. official said after Washington announced visa bans and asset freezes on 11 Russian and Ukrainian officials.

(This story has been refiled to say Monday in first paragraph)

(Reporting By Matt Spetalnick)