May 3 (UPI) -- Syria's political opposition on Wednesday suspended negotiations with President Bashar al-Assad's regime in Astana, Kazakhstan, due to ongoing airstrikes.
The fourth round of negotiations backed by Russia, Turkey and Iran began earlier on Wednesday. As the opposition High Negotiations Committee and the Syrian government discussed a Russian plan to de-escalate violence in certain areas, the opposition walked out -- citing regime bombings of rebel-held areas.
"Let them stop killing. You don't call on people for taking part in these talks, for cease-fire, and at the same time kill them. This is a war crime," HNC adviser Yahya al-Aridi told Russia's state-run Tass news agency.
Al Jazeera reported the HNC is not fully withdrawing but instead walked out of meetings and has not left.
Russia's proposed "de-escalation zones" would establish cease-fire areas with the hopes to allow civilians to return and where there would be humanitarian aid and medical assistance.
The rebel Free Syrian Army said the opposition suspended participation due to government attacks in the rebel-held Syrian cities of Nasib and Hama, as well as in the eastern part of Damascus.
"The opposition delegation withdraws from today's meeting due to continuous regime attacks, especially on Nasib and Hama," the FSA said in a statement.
Syria has been blighted by a complex civil war in which the Islamic State, the Syrian government and multiple Syrian rebel groups fight for control of territory.