The United States has strongly condemned the use of force in Libya and called on Tripoli to allow peaceful protests after "credible reports" of hundreds of casualties in a government crackdown.

"The United States is gravely concerned with disturbing reports and images coming out of Libya," said a written statement from State Department spokesman Philip Crowley made public Sunday.

"We are working to ascertain the facts, but we have received multiple credible reports that hundreds of people have been killed and injured in several days of unrest -- and the full extent of the death toll is unknown due to the lack of access of international media and human rights organizations."

Human Rights Watch earlier said it feared a catastrophe with more than 170 people dead in an iron-fisted crackdown in the flashpoint city of Benghazi, while a lawyer told AFP the death toll was greater than 200.

"We have raised to a number of Libyan officials, including Libyan Foreign Minister Musa Kusa, our strong objections to the use of lethal force against peaceful demonstrators," said Crowley.

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

The appeal came as intense gunfire was heard in the heart of Tripoli and several quarters of the city for the first time since the uprising began, but sounds of celebration also rang out to produce a confused picture.

The son of Libya's strongman Muammar Gaddafi warned the country would be destroyed by civil war if protests end his father's rule, in a speech broadcast as bursts of gunfire broke out in Tripoli.

Saif Al-Islam Gaddafi offered reforms to end the violent uprising, but he warned the protests were a foreign plot and would be crushed in a "bloodbath" if Tripoli's offer was rejected. He put the death toll at 84.

Libya's unrest has spread from the flashpoint city of Benghazi, where demonstrations began on Tuesday, to the Mediterranean town of Misrata, just 200 kilometres (120 miles) from Tripoli.

Witnesses described security forces, backed by "African mercenaries," firing into crowds "without discrimination," and a lawyer told AFP at least 200 people had been killed in the five days of unrest.

As tensions on the ground escalated, the US government authorized relatives of embassy staff to leave Libya on Sunday, amid growing violence against anti-regime protesters believed to have left at least 170 people dead.

In its travel warning, the State Department urged US citizens in Libya to exercise "extreme caution" when traveling, advising them against "non-essential" travel to eastern Libya.

"The US Department of State strongly urges US citizens to avoid all demonstrations, as even peaceful ones can quickly become unruly and a foreigner could become a target of harassment, or worse."

But the agency responsible for US diplomacy also acknowledged that "there is no indication that Westerners are being threatened or targeted."

Analyzing the roots of unrest in Libya and other Middle Eastern nations, US Republican Senator Richard Lugar, an influential member of the Senate Foreign Relations Committee, said that in most of these countries aging autocratic leaders were confronted by young people clamoring social and economic change.

"They know they are not getting their fair share, that life is not going to be good for them," Lugar told CNN television. "The question is, will, as in the case of the Libyans, the protesters simply be shot?"

Meanwhile, a UN spokesman said late Sunday that UN Secretary-General Ban Ki-moon intended to raise what the spokesman described as "very disturbing reports" emanating from that country with the Libyan leadership