RIYADH, Saudi Arabia -- Syrian President Bashar Assad has two choices, "either to leave through negotiations" or be forcibly removed from power, the Saudi foreign minister said on Thursday, arguing that the Syrian people would not accept any other outcomes.

Speaking to reporters during a two-day meeting of Syrian opposition groups taking place in the Saudi capital, Riyadh, Adel al-Jubeir said he hoped the various factions can come up with a common vision for Syria.

The meeting, which ends on Thursday, aims at forming a unified front ahead of proposed peace negotiations with Assad's government.

A peace plan agreed last month by 20 nations meeting in Vienna set a Jan. 1 deadline for the start of talks between Assad's government and opposition groups.

U.S. Secretary of State John Kerry, speaking at a climate conference outside Paris on Thursday, said the talks in Riyadh have been "very constructive."

"I think everybody is moving in the direction that they want to rapidly get to a political process and get it underway under U.N. auspices. So we made progress but we have some tough issues to get over," he said.

Kerry said that he spoke to al-Jubeir and Russian Foreign Minister Sergey Lavrov to organize the next Syria meeting, tentatively set for Dec. 18 at the United Nations headquarters in New York. He added that while he was "working toward" the U.S. meeting the arrangements were "not locked in yet."

Al-Jubeir said the Riyadh convention aimed to put the opposition in a "stronger position" by agreeing on shared principles for future peace negotiations. He added that he wanted to remove any opportunity for critics to accuse the Syrian opposition of being too fragmented and lacking a vision for the future.

Saudi Arabia has been a key backer of Sunni opposition blocs pushing for Assad's ouster throughout the nearly five year old Syria conflict. Among those participating in the meetings in Riyadh are hard-line Saudi-backed groups such as Jaysh al-Islam and Ahrar al-Sham, who had long rejected any negotiations with Assad's government while he remained in power.

The largest bloc at the meeting, with around 20 delegates, is the Western-backed opposition group known as the Syrian National Coalition. Also in attendance are representatives of the Syria-based National Coordination Body, an organization sometimes accused by other opposition members of being too conciliatory toward Assad's government.

Security was exceptionally tight at the hotel venue on Thursday. The hotel entrance was blocked off to the general public by armoured vehicles and dozens of security guards were stationed outside the building and in the lobby.

Al-Jubeir repeated Saudi Arabia's longstanding position that Syria's president must go.

"As I said before, Bashar Assad has two solutions: Leave through negotiations, which is easier and better for all. Or he will have to leave through fighting because the Syrian people refuse that this regime and person stays in power," he said.