A powerful Syrian insurgent faction pulled out of an opposition conference held in Saudi Arabia on Thursday in protest at the role given to groups it said are close to the Syrian government.

Ahrar al-Sham, a Saudi-backed ultraconservative group that operates mainly in northern Syria, said in a statement that it was withdrawing also because some of its comments and recommendations have been disregarded at the meeting.

The group's withdrawal came at the end of the two-day conference in the Saudi Arabian capital, Riyadh, which sought to form a unified opposition front ahead of proposed peace negotiations with Syrian President Bashar Assad's government.

Earlier, the U.S. Secretary of State John Kerry, speaking at a climate conference outside Paris, had described the talks in Riyadh as "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 told reporters that he had spoken to Saudi Foreign Minister 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."

Speaking at a news conference earlier on Thursday, Al-Jubeir said that Assad has two choices, "either to leave through negotiations" or be forcibly removed from power, arguing that the Syrian people would not accept any other outcomes. He said he hoped the various opposition factors could come up with a common vision for Syria.

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 armored vehicles and dozens of security guards were stationed outside the building and in the lobby.

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.