A U.S. State Department spokesman made it clear on Monday that Libyan leader Muammar Gaddafi's departure from power is "non-negotiable."

"We've said it's our bottom line, it's a non-negotiable demand," Mark Toner told reporters at the department, saying that "we believe he needs to depart power, he needs to step down, he's delegitimized as a leader."

But he acknowledged "a difficult situation" in Libya as the North African state is currently split in half.

An African Union delegation headed by South African President Jacob Zuma is mediating a political solution in Libya, and Gaddafi has agreed to its four-point peace plan which calls for ending fighting, facilitating humanitarian assistance, protecting foreign nationals and discussing political transition and reform.

U.S. Secretary of State Hillary Clinton reiterated on Monday that the United States wants to see a cease-fire, and the Libyan government forces "pull back from the areas that they have forcibly entered" and a resumption of water, electricity and other services to cities that "have been brutalized by the Gaddafi forces."

"We want to see humanitarian assistance reach the people of Libya. These terms are non-negotiable," Clinton told reporters after meeting Finnish Foreign Minister Alexander Stubb at the State Department.

"We believe, too, that there needs to be a transition that reflects the will of the Libyan people and the departure of Gaddafi from power and from Libya," she added.