US President Barack Obama said this evening that the death of Muammar Gaddafi ended a long, painful chapter for Libyans and warned "iron fist" regimes in the rest of the Arab world they would inevitably fall.

Speaking in the White House Rose Garden, Obama said Gaddafi's demise vindicated the collective military action of the West and said Libyans now had a chance to build a "democratic" and "tolerant nation."

"Just one year ago, the notion of a free Libya seemed impossible, but then the Libyan people rose up and demanded their rights."

"This is a momentous day in the history of Libya, the dark shadow of tyranny has been lifted," Obama said, in his first reaction to the killing of Gaddafi near his hometown of Sirte.

"With this enormous promise, the Libyan people now have a great responsibility to build an inclusive and tolerant and democratic Libya that stands as the ultimate rebuke to Gaddafi's dictatorship."

Obama said that Washington looked forward to an official announcement of Libya's liberation, which officials in Tripoli said could come as early as Friday.

The president, who was criticized by Republicans either for failing to take a more proactive role in the NATO operation in Libya, or for intervening at all, also had a warning for other dictatorial leaders in the Middle East.

"Across the Arab world, citizens have stood up to claim their rights. Youth are delivering a powerful rebuke to dictatorship. And those leaders who try to deny their dignity will not succeed," he said.

"For the region, today's events prove once more that the rule of an iron fist inevitably comes to an end."

Obama did not mention any nations by name, but his remarks appeared to be a clear reference to US foes Syria and Iran which have both violently put down movements for change.

The president also called on the new Libyan authorities to cooperate with the United States and its allies to track down munitions including surface-to-air missiles which went missing amid the revolutionary chaos.

The president also remember those Americans who had been killed in terror attacks orchestrated by Gaddafi's Libya, perhaps thinking especially of those who died in Pan Am Flight 103 over Lockerbie, Scotland in 1988.

"We are reminded today of all those Americans that we lost at the hands of Gaddafi's terror, their families and friends are in our thoughts and in our prayers," Obama said.

"We recall their bright smiles, their extraordinary lives, and their tragic deaths," he said.