United States Secretary of State John Kerry, on Sunday, accused Syria of “clearly trying to disrupt” the United Nations-sponsored peace talks aimed at ending the bloody five-year civil war in the country, by demanding that there be no discussion of removing President Bashar al-Assad from power.

The top U.S. diplomat said that violence in the war-wracked country has been “hugely reduced” by 80 to 90 per cent since a “cessation of hostilities” was declared two weeks ago. But he said the “single biggest violator” of the truce has been the Assad regime. “Aerial bombardments ... must stop,” Kerry said. “Look hard at who is committing these violations.”

He said “incremental violations threaten to undermine” efforts to permanently end the fighting and any effort to eventually hold elections in Syria. Kerry spoke after meeting with his British, French, German and Italian counterparts Sunday in Paris about the Syrian crisis, a day before the U.N. talks are set to begin in Geneva.

Ahead of Monday’s discussions, Syrian Foreign Minister Walid Muallem warned negotiators that any talk about the fate of Syria’s president is off the table. “We will not talk with anyone who wants to discuss the presidency . . . Bashar al-Assad is a red line,” Muallem said.  Mohammad Alloush, the chief negotiator for Syria’s main opposition group, said the president has to go, a demand the U.S. also has long made.

Alloush told the French news agency AFP, “We believe that the transitional period should start with the fall or death of Bashar al-Assad.”

Kerry has urged both sides in Syria  to proceed with the peace talks despite their conflict over the presidency.

Muallem said the Syrian government remains committed to the cease-fire agreement, but its delegation to the peace talks will only wait 24 hours for the opposition delegation to arrive for the talks. Muallem said Saturday in Damascus the diplomats will leave for Geneva Sunday.