No Assad in transitional govt: Kerry

Syrian President Bashar al-Assad will not be part of any new transitional government, US Secretary of State John Kerry said at the start of a peace conference.

"We need to deal with reality here ... mutual consent, which is what has brought us here for a transitional government, means that that government cannot be formed by someone that is objected to by one side or not," Kerry said in an opening statement.

"That means that Bashar al-Assad will not be part of that transitional government.

"There is no way, not possible in the imagination, that the man who has led the brutal response to his own people could regain legitimacy to govern."

Efforts to broker a peace settlement for Syria will be neither simple nor quick, but a peace conference marks a historic opportunity, Russian Foreign Minister Sergei Lavrov said.

"The negotiations will not be simple, they will not be quick," Lavrov said in the Swiss lakeside town of Montreux.

"There is a historic responsibility on the shoulders of all participants," he told delegates from Syria's warring sides and some 40 countries.

"Our common task is to achieve an end to the tragic conflict in Syria, which has brought incalculable pain to the Syrian people."

Russia is the leading world power ally of Syrian President Bashar al-Assad, who supporters of the three-year Syrian revolt say no longer has a place at the country's helm.

While backing calls for a Syrian transition government - a plan agreed to by the international community at a 2012 conference in Geneva, but never implemented - Lavrov warned against imposing Arab Spring-style political change from outside.

"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 modernisation," he said.

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

Lavrov also warned of the rising ramifications of the Syrian crisis beyond the country's borders.

The Syrian war has become a magnet for foreign Islamist fighters, leading the Assad regime to depict the conflict as a "war on terror", although the opposition fighters have also clashed with some jihadist hardliners.

"The threat of Syria turning into a cauldron of international terrorism has turned into a most serious problem. Extremists from across the world are wrecking chaos, ruining cultural and democratic foundation formed over centuries," said Lavrov, whose comments were translated from Russian into English.