The site of an explosion at a Libyan Foreign Ministry building in Benghazi Wednesday. Esam Omran Al-Fetori/Reuters

A powerful blast has caused severe damage to a foreign ministry building in the eastern Libyan city of Benghazi, witnesses said.

It was not immediately known if Wednesday's blast resulted in casualties.

The explosion came on the first anniversary of an attack by armed men on the U.S. consulate in Benghazi, which killed four Americans, including Ambassador Chris Stevens.

The U.S. government initially said the assault grew out of anti-Western protests. Later, however, it turned out that an armed group launched the attack on the anniversary of the Sept. 11 terrorist attacks.

Benghazi, the cradle of the 2011 revolt that toppled and killed dictator Muammar Gaddafi, has been hit by a wave of deadly attacks in recent months targeting security force officers and members of the judiciary, many of whom served in the previous regime.

Attacks have also targeted diplomats and Western interests.

Much of the violence, including the killing of the U.S. ambassador last year, has been attributed to Islamists.

Documents obtained exclusively by Al Jazeera's Investigative Unit revealed that the U.S. State Department knew of the security problems in Benghazi, but failed to fix them.

Al Jazeera and wire services