WASHINGTON, Dec 12 (Reuters) - The Pentagon said on Wednesday that any unilateral action into northeast Syria was of grave concern and "unacceptable," after Turkey announced that it would launch a new military operation in the region within days to target Kurdish militia fighters.

"Unilateral military action into northeast Syria by any party, particularly as U.S. personnel may be present or in the vicinity, is of grave concern. We would find any such actions unacceptable," Commander Sean Robertson, a Pentagon spokesman, said in a statement.

Robertson said that the United States was committed to Turkey's border security but that the fight against Islamic State was not over and the Syrian Democratic Forces remained a "committed partner" against the militant group. (Reporting by Idrees Ali; Editing by Sandra Maler)

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