DUBAI - A senior aide to Iran's supreme leader warned against the overthrow of Syria's President Bashar Assad, saying his fate was a "red line", in one of the Islamic state's strongest messages of support for the Damascus government.Iran has steadfastly backed Assad's rule since an uprising against his rule began almost two years ago and regards him as an important part of the axis of opposition against arch-foe Israel.Iran, a regional Shi'ite Muslim power which backs Lebanon's Hezbollah group, describes many Syrian opposition groups as "terrorists" who are backed by Western and Arab states. Assad follows an offshoot of Shi'ite Islam.Velayati blamed what he called "reactionary" Arab states for the violence in Syria and singled out Qatar, accusing it of bringing in fighters from Somalia and Afghanistan to help topple Assad.Velayati said all parties linked to the crisis in Syria needed to negotiate."Anyone who comes to the talks cannot negotiate on the table and support the armed elements, but must enter the negotiations and stop supporting the armed elements," he added.The Islamic Republic has sought international backing for its six-point plan to resolve the Syrian conflict. The plan calls for an immediate end to violence and negotiations between all parties to form a transitional government, but does not call for Assad to step down."If the Syrian President Bashar Assad is toppled, the line of resistance in the face of Israel will be broken," Ali Akbar Velayati, who is seen as a potential contender in Iran's June presidential election, said in an interview broadcast on Sunday."We believe that there should be reforms emanating from the will of the Syrian people, but without resorting to violence and obtaining assistance from the (United States of) America," he told Lebanon's Al-Mayadeen satellite television.Asked if Iran sees Assad as a red line, Velayati said: "Yes, it is so. But this does not mean that we ignore the Syrian people's right in choose its own rulers."More than 60,000 people have died in the uprising against Assad, part of the Arab Spring protests that have swept aside four heads of state since 2011.