"We see only one option, negotiating a transition government born by mutual consent," Kerry said in his opening statement during the Geneva II conference on the Syrian crisis on Wednesday.

"That means that Bashar Assad will not be part of that transition government,” the top US diplomat claimed.

The development comes as representatives from the Syrian government and members of the country's foreign-backed opposition gathered in Montreux, Switzerland, for negotiations aimed at ending Syria's three-year unrest.

Syrian Foreign Minister Walid al-Muallem, however, rejected Kerry's assertion during his speech at the conference, insisting that only Syrians have the right to choose their president.

"No one, Mr. Kerry, no one can grant or withdraw the legitimacy of the president other than the Syrians themselves," he said. "This is their right and duty."

Russian Foreign Minister Sergei Lavrov also said that outsiders must not meddle in the Arab country’s internal affairs.

"Attempts to dictate to countries of the Middle East and North Africa recipes of reforms from the outside, to set up experiments in social engineering, this only throws back the processes of political and economic modernization," Lavrov said.

"You do not need to go far to find the relevant examples," he added.

At the conference, UN Secretary-General Ban Ki-moon said the talks face "formidable" challenges.

"We know that it has been an extremely difficult path to reach this point. We have lost valuable time and many many lives. Let me not mince words, the challenges before you and before all of us are formidable. But your presence here raises hope," he said.

Syria has been gripped by deadly unrest since 2011. According to reports, Western powers and their regional allies - especially Qatar, Saudi Arabia, and Turkey - support the militants operating inside the country.

HH/MB