'He was a role model to those who worked with him and to the young diplomats who strive to follow in his footsteps.
These four Americans stood up for freedom and human dignity. We grieve with their families but let us carry on their memory... I have no doubt that their legacy will live on.'
He added: 'The United States condemns in the strongest possible terms this outrageous and shocking attack. There is no justification to this type of senseless violence. None.'
The deaths will put huge pressure on the Libyan administration which took over after the fall of Muammar Gaddafi.
Despite the West taking a key role in his overthrow, the country remains unstable and Islamic militants have moved into the power vacuum.
There are also huge questions about the security surrounding both Ambassador Stevens, who was believed to be in Benghazi for the opening of an American building, and the US diplomatic buildings in such a volatile country.
