Syrian rebels launched a fresh wave of attacks on government-controlled western Aleppo on Thursday, killing 12 civilians and wounding 200 according to state media, one day before a 10-hour "humanitarian pause" unilaterally declared by Moscow was set to take effect.

Meanwhile, an airstrike on a rebel-held village south of the contested city killed at least nine civilians, opposition activists said.

The city of Aleppo and its surrounding environs have become one of the main theaters of the Syrian war. It is the country's largest city and its former commercial capital and represents a major prize for any side that can claim control over it after more than five years of war. The rebels control the city's eastern districts, which have been besieged by pro-government forces since July.

Social media accounts linked to the rebels carried photos and footage purporting to show the factions targeting government positions with artillery, tank and rocket fire. Rebels detonated an explosives-laden vehicle in a suicide operation targeting the government-held New Aleppo district, according to the al-Qaida-linked Fatah al-Sham Front, which circulated footage of the blast on its social media accounts.

The Nour el-Din el-Zinki movement said rebels detonated two other vehicle bombs, as well, including one on the military's academy in the west.

Ahrar al-Sham, another rebel group, said they had advanced into a western, government-held neighborhood known as the 3000 Apartments complex, a claim that could not be independently verified.

Syrian state media said 12 civilians were killed by rebel fire and eight others hospitalized with breathing difficulties when rebels targeted several government-controlled neighborhoods with poison gas. The claim could not be independently verified.

The attacks came as a Russian lawmaker pledged that Russian and Syrian government forces would "purge" the city if rebels do not leave its eastern neighborhoods by the end of the "humanitarian pause" in fighting due to take place between 9 a.m. and 7 p.m. on Friday.

Earlier this week, the Russian and Syrian militaries promised a pause in fighting and to open humanitarian corridors out of eastern Aleppo. On Wednesday, Russian President Vladimir Putin urged rebels to use the passages to leave the city.

Senator Frants Klintsevich, who carries little political weight, told the Interfax news agencies on Thursday that a "purge will begin" if "there are no results" once the pause ends. He added, however, that saving civilians is the main priority.

Other Russian officials have not indicated what they intend to do when the pause ends. Russia recently dispatched aircraft carrier Admiral Kuznetsov, which is now in the Mediterranean and heading for the Syrian coast. Its presence suggests that Russia may be intending to escalate its assault on rebel-held parts of the northern Syrian city.

Meanwhile, tension between two of the largest rebel factions in Aleppo continued Thursday, a day after clashes between the two groups broke out. The development bodes ill for the rebels' ability to battle pro-government forces, although Yasser al-Yousef of Nour el-Din el-Zinki group said his faction and the Fastaqim group had agreed to arbitration on Thursday.

However, in a sign of continued tensions, fighters aligned with the hard-line Conquest Army opposition coalition, which includes the el-Zinki faction, seized the headquarters and warehouses of the more moderate Fastaqim group, according to a media official for Fastaqim.

The U.N. estimates 275,000 people, the vast majority civilians, are trapped inside rebel-held eastern Aleppo with dwindling food and supplies. U.N. officials have condemned the tactic, employed primarily by the government across the country, as "medieval," ''barbaric," and in contravention to international law.

The U.N. says more than one million people are residing in the government-controlled west. Many are displaced families from the east.

The activist-run Local Coordination Committees elsewhere reported that airstrikes on the village of Miznaz, southwest of Aleppo, had killed nine civilians. It blamed the strike on Russian aircraft. The Observatory said that 10 civilians had died, among them seven children.