On the eve of a key meeting, US Secretary of State John Kerry Thursday made a powerful plea to the divided Syrian opposition to join landmark peace talks aimed at installing a new government.

Next week's peace conference in Switzerland was "the best opportunity for the opposition to achieve the goals of the Syrian people and the revolution," Kerry said in a surprise statement to reporters.

Despite months of cajoling and negotiations, the Syrian opposition has yet to agree to sit down at the table with members of the Syrian regime of President Bashar al-Assad to chart a path to end the war.

The UN-led peace conference, to be attended by some 35 countries, will open on January 22 in the Swiss city of Montreux, and then move to Geneva.

It will be the first time that the two sides have come together since the conflict erupted in March 2011, unleashing a brutal war which has claimed over 130,000 lives, and created millions of refugees.

Complicating the situation is the presence of extremist groups which flooded into Syria, leaving the more moderate US-backed opposition fighting both Assad's forces and Islamic militants.

Kerry, who only returned early Thursday from an overseas trip during which he attended a Syria donor's conference in Kuwait, stressed the US was "deeply concerned about the rise of extremism."

"The world needs no reminder that Syria has become the magnet for jihadists and extremists. It is the strongest magnet for terror of any place today," he warned.

The Syrian Opposition Coalition is due to vote Friday in Istanbul, and Kerry sought to ally their fears that the Geneva talks will somehow legitimize Assad's regime and leave him clinging to power.

A key bloc in the Coalition, the Syrian National Council, has however threatened to pull out, if the General Assembly votes in favor of attending.

The aim of the talks is to find a way to install a transitional government -- as agreed to in a June 2012 deal known as Geneva I.

The opposition can veto any names put forward for the transitional governing body, as does the regime, the top US diplomat stressed.

"Any names put forward for leadership of Syria's transition must, according to the terms of Geneva I ... those names must be agreed to by both the opposition and the regime," he said.

"This means that any figure that is deemed unacceptable by either side, whether President Assad or a member of the opposition cannot be a part of the future," Kerry added.

Thursday's statement was just the latest bid by the US administration to pressure the Syrian opposition to attend the conference, which has been in the planning since May, with US ambassador to Damascus Robert Ford having spent months huddled in talks to bring them together.

At the weekend Kerry also met with opposition National Coalition leader Ahmad Jarba in Paris.

Although he did not make a firm commitment to attend the talks dubbed Geneva II, Jarba did indicate he had been reassured by the tone of Sunday's discussions.

"We all agreed that there is no future for Bashar al-Assad and his family in Syria," he said. "His departure is inevitable."

Kerry insisted Thursday: "The Syrian people need to be able to determine the future of their country, their voice must be heard."

"It defies logic to imagine that those whose brutality" had allowed extremists to flood into the country "could ever lead Syria away from extremism and towards a better future," he insisted, reiterating the US position that Assad cannot play any role in a future Syrian government.