Bad weather was preventing Thursday two Maltese ferries from evacuating hundreds of US citizens from a port near the Libyan capital Tripoli, officials said, DPA reported.
"The ferry departure from Libya to Malta is delayed due to high seas. Citizens are safe on board. It will leave when the weather permits," US State Department spokesman Philip Crowley said on the social-networking site Twitter.
Around 575 American citizens, including diplomats, are expected to spend the rest of the day sheltering in As-Shahab harbour as heavy seas persist in the central Mediterranean.
The two vessels - normally used to transport holidaymakers between Malta and Sicily but chartered by US authorities for the evacuation - were due to leave Libya on Wednesday evening.
The State Department said the Libyan authorities had been "cooperative" during the embarkation of the US citizens at As-Shahab.
The seaborne operation was rendered necessary after Libya denied landing rights for US aircraft in Tripoli.
A revolt against Libyan leader Moamer Gaddafi in which hundreds are believed to have been killed by government forces has prompted Western nations to begin evacuating their citizens.
The small Mediterranean island nation of Malta is geographically the closest European Union member state to Libya.