"The United States is gravely concerned with disturbing reports and images coming out of Libya," State Department spokesman P.J. Crowley said in a statement.
He said the US was still "working to ascertain the facts", but had received "multiple credible reports" of hundreds killed in recent unrest.
US officials had spoken with Libyan officials, including Foreign Minister Musa Kusa, to convey strong objections to violence directed at peaceful demonstrators.
Protests against Libyan leader Muammar Gaddafi continued Sunday for the sixth straight day, with the flashpoint city of Benghazi in the north-east reporting the highest death toll.
With journalists barred from travelling to Benghazi, and internet and mobile phone connections reportedly cut by the government, obtaining reliable information about the situation in the country ruled by Gaddafi since 1969 is difficult.
But more than 200 people had been killed over the past two days, a doctor in Benghazi city told Al Jazeera television, after protests had turned violent Friday.
Published 21 February 2011, 04:05 IST