Information Minister Omran al-Zoubi said in an interview with Lebanese al-Manar TV that his country was not willing to discuss the fate of the president, the form of government and the constitution, all of which are "the essence of sovereignty".
Only the Syrian people can decide on the form of government "through ballot boxes", he added.
Syria as "a sovereign democratic, pluralistic country" cannot be divided, the minister said.
Russian Foreign Minister Sergei Lavrov said after negotiations with US Secretary of State John Kerry that the two countries had decided to hold an international conference at the end of May, aimed at facilitating a solution to the Syrian crisis through political dialogue.
Russian President Vladimir Putin met British Prime Minister David Cameron in Sochi last week for talks on Syria. Cameron is currently in the US for further talks on Syria with US President Barack Obama.
About 70,000 people have died in Syria since the beginning of the uprising against Assad in March 2011, according to UN figures.