Russian and Syrian forces bombed a strategic camp on the northern edge of Aleppo on Sunday after losing control of it overnight, both rebels and the Syrian army said.
Rebels said the army used more powerful weapons in an attempt to recover Handarat, a Palestinian refugee camp a few kilometers north of the Aleppo which is on elevated ground overlooking one of the main roads into Aleppo that had been in rebel hands for years.
"We retook the camp, but the regime burnt it with phosphorous bombs … We were able to protect it, but the bombing burnt our vehicles," said Abu al-Hassanien, a commander in a rebel operations room that includes the main brigades fighting to repel the army assault.
The army which is being helped by Iranian-backed militias, Lebanon's Shi'ite Hezbollah militant group and a Palestinian militia, acknowledged rebels had retaken Handarat, whose capture on Saturday briefly marked the first major ground advance by the army in a new offensive to recapture rebel held Aleppo.
"The Syrian army is targeting the armed groups positions in Handarat camp," a military source was quoted on state media as saying.
The army announced on Thursday the start of a major new military campaign to regain Aleppo, intensifying strikes and use of powerful weapons that rebels said was an "shock and awe" campaign aimed at creating devastation and demoralizing them.
Russian planes also continued their pounding of residential parts of Aleppo, with whole buildings flattened, according to rebels and residents.
The assault on Aleppo, where more than 250,000 civilians are trapped, could be the biggest battle yet in a civil war that has killed hundreds of thousands of people and driven 11 million from their homes.
Two weeks after Moscow and Washington announced a ceasefire that quickly unraveled, President Bashar al-Assad and his Russian and Iranian allies appear to have launched a campaign for a decisive battlefield victory that has buried any hope for diplomacy.
