WASHINGTON, Feb 12 (Reuters) - A senior U.S. government official warned pro-Russian separatists on Thursday against seizing more territory in eastern Ukraine before a ceasefire agreement goes into effect on Sunday.

"We are trying to send the message as strongly as we can that any effort to grab more land between now and Saturday night ... will seriously undercut this agreement and to warn against it in the strongest terms," the official, who spoke on condition of anonymity, told reporters on a conference call. (Reporting By Arshad Mohammed; Editing by Sandra Maler)

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