BEIRUT, March 6 (Reuters) - The U.S. embassy in Lebanon has urged its citizens residing there to keep a low profile, citing concern that "extremist groups" may be planning attacks against Americans.

"There is a general concern that extremist groups may be planning to attack U.S. citizens and interests in Lebanon," the U.S. embassy said in a statement, dated March 4, posted on its Web site and circulated to U.S. citizens.

The United States backs the Beirut governing coalition which is locked in a power struggle with an opposition led by Hezbollah -- a faction supported by Syria and Iran which Washington lists as a terrorist group.

The United States has deployed three warships in the eastern Mediterranean off the coast of Lebanon to send a signal to Syria, which Washington accuses of meddling in the country.

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