Assad's status is a "red line," Syrian Foreign Minister Walid Moallem told reporters in Damascus, even as he confirmed that regime negotiators were prepared to attend Monday's peace talks in Switzerland.
"They need to get rid of these illusions," Moallem said of the rebels. "We will not negotiate with anyone on the presidency. Bashar is (a) red line and belongs to the Syrian people."
The U.N.'s special envoy for Syria, Staffan de Mistura, had said Wednesday that the talks would focus on certain main points, including how to govern Syria, a new constitution, and presidential elections.
Moallem's comments came as the United Nations prepared to host negotiations between the Syrian regime and a main opposition group in Geneva on Monday, more than two weeks into a shaky truce that was designed in part to allow for the talks.
Kerry: 'Serious consequences'
On Sunday, U.S. Secretary of State John Kerry called Moallem's comments a "message of disruption" and decried violations of the truce by the Syrian government.
"If the regime and its backers think that they can test the boundaries, diminish compliance in certain areas, or act in ways that call into question their commitment to the cessation, without serious consequences for the progress that we have made, they are deeply mistaken," Kerry said at a news conference.
He called on Russia and Iran, both major backers of the Assad government, to make sure it lives up to its commitments under the peace process.
A main opposition group called the High Negotiations Committee had said before Moallem's comments that it would attend the Geneva talks. Reaction from the HNC to Moallem's comments wasn't immediately available.
But its lead negotiator, who has repeatedly accused the regime and its allies of violating the truce, has already said he doubted anything would come of the sessions.
The rebel group demands, among other things, that Assad have no role in any government that would come from the peace process.
Both sides claim truce violations
Riyad Hijab, the HNC's lead negotiator, on Friday played down the chances of a deal, "given that the regime is continuing to exploit the suffering of the Syrian people to avoid inevitable outcomes of the political process."
Past efforts to negotiate an end to the conflict have been hindered by strong opposition to the Assad regime, which is perceived as brutal, and by the fractured nature of the opposition. Groups fighting the regime include not only rebels represented by the HNC, but also many others that are not part of the talks, including the terror group ISIS, which has seized parts of Syria and Iraq.
Last week, Hijab said that the truce was "close to collapse" because of combat attacks against the opposition by the Syrian regime and its allies, including Russia, which is conducting airstrikes in the country on the Assad regime's behalf.
Syria and Russia agreed to a truce with only some of the groups fighting in the Middle Eastern country. A number of combatants, including ISIS, are not covered by the agreement.
Each side has accused the other of violating the truce, which also is meant to allow humanitarian aid to reach people who have been cut off by the fighting.
U.N. 'road map' called for elections, transition
The U.N.-sponsored talks are part of a "road map" for Syrian peace contained in U.N. Security Council Resolution 2254
, which the council adopted in December.
The resolution calls for a political transition aimed at establishing "a credible, inclusive and nonsectarian governance"
in Syria. It also foresees that negotiations will result in a new constitution and transparent elections.
Chief diplomats from the United States, Britain and some other Security Council nations have argued that Assad has lost the credibility to lead his country and must leave office as part of any peace deal. But the resolution, while calling for an inclusive political transitional governing body with full executive powers, doesn't mention or explicitly exclude Assad.