Ronald Smith, an American chemistry teacher working at the International School Benghazi, was shot and killed on Thursday, a school official confirmed to CBS News. "He was doing his morning exercise when gunmen just shot him. I don't know why. He was so sweet with everyone," said Adel al Mansouri, the director of the school told Reuters. 

It was not immediately clear who was responsible for the killing, but the attack came just a few days after an Al-Qaeda leader encouraged supporters to take revenge against Americans. A 17-minute audio message was released on Jihadi blogs on Sunday by Adam Yaha Ghadahn, an American-born Al Qaeda leader, encouraging attacks on Americans. 

"The untimely death of Ronnie Smith has been felt by the whole school community. He was a much loved teacher who supported students in their learning and always had time to help when asked. Ronnie was a professional who gave his time freely and without question. We do not understand why this has happened and it is extremely difficult for his students and his colleagues to accept," read a statement on Facebook.

Security sources also said that three soldiers were killed in separate attacks on Thursday in Benghazi. This marks the second big attack on Americans in Benghazi, Libya.

Ansar al-Sharia terrorists attacked the U.S. Consulate on September 11, 2012, killing Ambassador J. Christopher Stevens and three other American diplomatic personnel.

Armed forces in Libya continue to battle with former fighters of ousted leader Muammar Qaddafi, as well as Islamist militants who control part of the oil-producing country, The AP reported.