Stalls are seen on a street beside damaged buildings in the rebel held al-Shaar neighborhood of Aleppo, Syria, February 10, 2016.

The Saudi-backed Syrian opposition on Wednesday said it “prioritized” a political solution for the bloody conflict marring the country but also said “there needs to be a balance of power on ground” after President Bashar al-Assad’s regime made advances with Russian and Iranian support, one of its top officials said.

Riyad Hijab, who was a former Syrian prime minister and who now works as coordinator for High Negotiations Committee, said Russia’s air raids is “guaranteeing Assad to stay in power,” during a press conference in London, UK.

He added: “We reject giving Assad and clique any immunity from crimes they committed,” describing Assad regime’s practices as being boon to radical militants.

Before Hijab’s statement, the opposition said on Wednesday that it will participate in peace talks in Geneva scheduled on Feb. 25, Al Arabiya News Channel reported.

The continued bombardment of rebel-held towns was among the reasons given by the Syrian opposition for not cooperating with the U.N. peace talks early this month.

The opposition also asked Washington to pressure Moscow to halt its airstrikes in Syria after incidents of high number of casualties.

A monitor said on Wednesday that more than 500 people, including dozens of civilians, have been killed since a major Russian-backed regime offensive in Syria’s Aleppo province began this month.

The Syrian Observatory for Human Rights, which relies on a network of sources for its figures, said the toll of 506 included 23 children killed in Russian air strikes on Aleppo city and its surroundings since the operation was launched on February 1.

The Syrian opposition request comes after U.S. Secretary of State John Kerry warned that the Russian air raids were killing women and children.

Meanwhile, Russian Foreign Minister Sergei Lavrov said on Tuesday Russia had proposed a “concrete” plan to resolve the Syria crisis to the United States, RIA news agency reported.

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