"We can confirm that our office in Benghazi, Libya, has been attacked by a group of militants. We are working with the Libyans now to secure the compound," Nuland said in a statement. "We condemn in strongest terms this attack on our diplomatic mission."
The Libyan Interior Ministry said one American was killed in the attack, Foreign Policy magazine reported.
The attack followed one Tuesday at the U.S. Embassy in Cairo in which protesters tore down the American flag and replaced it with the flag of al-Qaida.
A mob of as many as 2,000 people had surrounded the embassy in Egypt, but only five or six individuals had breached the compound, Nuland said.
"In Cairo, we can confirm that Egyptian police have now removed the demonstrators who had entered our embassy grounds earlier today," she said.
Media reports said the release of a film titled "Muhammad, Prophet of the Muslims" sparked the protests in Egypt and Libya.
"The Embassy of the United States in Cairo condemns the continuing efforts by misguided individuals to hurt the religious feelings of Muslims -- as we condemn efforts to offend believers of all religions," the embassy said earlier in a statement. "Today, the 11th anniversary of the September 11, 2001 terrorist attacks on the United States, Americans are honoring our patriots and those who serve our nation as the fitting response to the enemies of democracy. Respect for religious beliefs is a cornerstone of American democracy. We firmly reject the actions by those who abuse the universal right of free speech to hurt the religious beliefs of others."
A State Department official said the government cannot confirm whether there is a connection between the incidents in Egypt and Libya.