WASHINGTON (Reuters) - The United States sees the rebel offensive for Tripoli as clearly under way, and is urging the National Transitional Council (TNC) to start planning for the post-Gaddafi era in Libya, the U.S. State Department said on Sunday.

"Clearly the offensive for Tripoli is under way," State Department spokesperson Victoria Nuland said in a statement.

"We continue efforts to encourage the TNC to maintain broad outreach across all segments of Libyan society and to plan for post-Gaddafi Libya. Gaddafi's days are numbered. If Gaddafi cared about the welfare of the Libyan people, he would step down now."