Syria's main opposition bloc has threatened to pull out of United Nation's-sponsored peace talks in Geneva if crimes of Bashar al-Assad's government persisted.

Representatives of the group, known as the High Negotiations Committee (HNC), landed in the Swiss capital late on Saturday, a day after a delegation representing the Assad regime arrived and held preliminary talks with the UN's Syria envoy Staffan de Mistura.

Speaking to Al Jazeera in Geneva late on Saturday, HNC spokesman Salem al-Meslet said: "We want the peace talks to work, but there is no seriousness on the part of the regime."

Meslet said that the HNC would discuss with de Mistura on Sunday its conditions for joining the negotiations, which include the government regime to lift sieges on opposition-held areas and stop shelling them, and agreeing to release prisoners.

"The priority is to lift the suffering of the Syrian people," he said. "If we see an implementation of these demands, including allowing trucks of aid into besieged areas, we will consider this as a sign of good will".

HNC coordinator Riad Hijab, who did not travel to Geneva, said in an Arabic statement posted online that "if the regime insists on continuing to commit these crimes then the HNC delegation's presence in Geneva will not be justified."

"The delegation will inform de Mistura of its intentions to withdraw its negotiating team if the UN and world powers are unable to stop these violations," Hijab wrote.

Monzer Makhous, a member of the Syrian opposition delegation, said: "We are here to test the intentions of the regime ... We do not have assurances, we have promises".

The proposed intra-Syrian talks are part of a peace plan set out in November by external powers embroiled in the five-year-old regime crackdown.

PHOTO CAPTION

Salem al-Meslet, spokesman of the Higher Negotiations Committee, talking to the press in Geneva

Al-Jazeera