The restive Iraqi city of Fallujah was sealed off by U.S. troops Friday and U.S. military officials vowed a major response to this week's killing and mutilation of four U.S. civilian contract employees.

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As of Friday morning, Fallujah appeared off-limits to Westerners.

Signs of a U.S. operation against militants in the city increased, however, and U.S. troops appeared to be cordoning off Fallujah and its outlying regions.

In the past, military sources have told United Press International that planning a significant military operation takes at least 48 hours and under ideal circumstances it can take longer. U.S. military officers and civilian authorities have promised to "pacify" Fallujah in response to Wednesday's attack.

There were also indications Friday that anti-coalition forces were preparing for a fight in the city. One source with close ties to the anti-U.S. resistance forces told UPI fighters from throughout the region had begun slipping into Fallujah in preparation for what they assume will be a significant U.S. military operation.

One such fighter -- who normally operates in Baghdad told UPI Friday he would leave immediately to join a cell in Fallujah and that fighters were also entering the city from the neighboring city of Ramadi.

"Fallujah is where we fight the invaders," a source familiar with the workings of the resistance told UPI through an intermediary Friday. "Ramadi is where our leadership and operations are based. They are moving (resources) into Fallujah for this fight. We will make Fallujah the graveyard of America."