WASHINGTON  The Obama administration on Sunday condemned Libya’s use of lethal force against peaceful demonstrators, pointing to what it said were “multiple credible” reports that “hundreds of people” had been killed and injured in several days of unrest.

In the administration’s strongest statement on the escalating violence in Libya, the State Department said that it was “gravely concerned” about the reports and that the number of deaths was unknown because of a lack of access to many parts of the country by news organizations and human rights groups.

Philip J. Crowley, the State Department spokesman, said that the United States has raised “strong objections about the use of lethal force” with several senior Libyan officials, including Musa Kusa, the foreign minister.

“Libyan officials have stated their commitment to protecting and safeguarding the right of peaceful protest,” Mr. Crowley said in a statement. “We call upon the Libyan government to uphold that commitment and hold accountable any security officer who does not act in accordance with that commitment.”