US State Department officials said late on Sunday they were seeking details of the raid that occurred earlier in the day.
According to a senior State Department official, the embassy complex remains secure, but the situation is very fluid as fierce clashes have been underway between rival militias for weeks.
Militiamen had captured a US embassy annex but had not broken into the main compound, according to reports.
The State Department official said the embassy was closed in July due the fighting between militias close to the compound. US personnel were evacuated with a military escort.
The action was taken in the light of controversy over an attack on the US consulate in Benghazi in 2012 which left four Americans dead, including ambassador Christopher Stevens.
Nearly three years after the overthrow of long-time dictator Muammar Gaddafi, Libya is still grappling with rising insecurity as the country has been witnessing numerous clashes between government forces and rival militia groups.
The former rebels refuse to lay down arms despite efforts by the central government to impose law and order.
Meanwhile, conflicting reports coming out of Libya, where fierce clashes have been underway between rival militant groups for weeks, indicate that Tripoli has fallen to the Fajr (Dawn) Libya gunmen.
Libya’s caretaker government has announced that most ministries and state-run institutions are now out of its control.
Attacks and looting have broken out across the city with the interior and electricity ministries and the prime minister's office ransacked.