Libyan medical officials say three people were killed as mortar shells slammed into a rally in the eastern city of Benghazi where hundreds had gathered in support of a military offensive on the Islamic State-held central city of Sirte.

Khalil Gwaidar, spokesman of Benghazi medical center, says three rockets hit an area near the al-Keish city center where the rally was taking place Friday.

Gwaidar and another medical official in Jalaa Hospital said three people were killed and 20 were wounded. Ambulances were seen rushing to the place.

The second official spoke on condition of anonymity because he is not authorized to talk to the media.

Libya's eastern forces, under army chief Khalifa Hifter, have announced a push on Sirte to retake it from IS.