But he warned it was "impossible to say" how long it would take to unseat the dictator.
Mr Hague also said his preference was for Col Gaddafi to be charged with crimes against humanity, rather than killed in an airstrike or exiled.
He said: "Time is on our side, not on his side. The position of the Gaddafi regime is weakening all the time.
"They have been losing important battles ... We have to have the patience and persistence to see this through to success, and remember that the time pressure is on the other side, on the Gaddafi regime."