The Pentagon said it was moving forces to “provide for that flexibility once decisions are made”.
The US already has a significant presence close to Libya, with several bases in southern Italy.
US Secretary of State Hillary Clinton has said US forces could be used for delivering humanitarian assistance.
Meanwhile, the US ambassador to the UN, Susan Rice, told reporters in Washington the US was “actively and seriously” considering establishing a no-fly zone and was in talks with Nato and other potential military partners.
Repositioned US forces could be used to enforce a no-fly zone to prevent Col Gaddafi’s aircraft attacking opposition supporters, BBC correspondents say.
“We have planners working various contingency plans, and I think it’s safe to say as part of that we’re repositioning forces to provide for that flexibility once decisions are made,” Pentagon spokesman Col Dave Lapan said.
The BBC’s Andrew North, in Washington, says the Pentagon’s announcement seems partly designed to send a message to Col Gaddafi.
It is still not clear if there will be sufficient support at the United Nations Security Council for a no-fly zone, our correspondent says.
US commanders could turn to the USS Enterprise, currently in the Red Sea, as well as the amphibious ship the USS Kearsarge, which has a fleet of helicopters and about 2,000 Marines aboard, AFP news agency reported.
In addition, the US maintains a large naval air station in Sigonella, Sicily, less than an hour’s flight from Libya.
Mrs Clinton said on Monday that the US was leaving all its options on the table in dealing with Libya.
Although she did not discuss military options, Mrs Clinton said that as long as Col Gaddafi remained in power the US would consider a range of options against Libya’s rulers.
“Through their actions, they have lost the legitimacy to govern. And the people of Libya have made themselves clear: it is time for Gaddafi to go – now, without further violence or delay,” she told the UN Human Rights Council in Geneva.
On Sunday, the New York Times reported that Western nations were looking at setting up a humanitarian “corridor” in neighbouring Tunisia or Egypt to help refugees.