At least 40 civilians were killed on Sunday in tit-for-tat attacks by Syrian regime forces and rebels in the northern city of Aleppo, a monitoring group reported.
The dead included 32 people who were killed in regime airstrikes and shelling targeting opposition-held districts in Aleppo, the Syrian Observatory for Human Rights added.
Eight others were killed in shelling attacks by rebels on regime-controlled areas in the city.
A total of 519 civilians, including 108 children have been killed in retaliatory attacks by the regime and rebels since April 22, said the Britain-based watchdog that relies on a network of activists inside Syria for information.
Once Syria's commercial hub, Aleppo is now divided between the government-held west and the rebel-controlled east.
The Observatory also reported that Kurdish-led Syrian forces were Sunday closing in on the city of Minbij, the centre of the Islamic State extremist group's last stretch of territory along the Turkish border.
The Kurdish-led Democratic Forces of Syria (DFS) were reportedly within 5 kilometres of the city's southern outskirts after six days of fighting.
Minbij is strategically important as it controls a supply route from the Turkish border to Syria’s north-eastern city of al-Raqqa, Islamic State’s de facto capital.
The Hawar news agency, which is close to Kurdish forces in the DFS, reported that the commander of one of the alliance's units, the Northern Sun Brigades, died Sunday from injuries sustained during fighting east of Minbij two days earlier.
The commander, known by his nom de guerre Abu Layla, was the most senior DFS leader to have been killed in fighting with Islamic State since the alliance was formed in October, according to the Observatory.
The monitoring group said Abu Layla had also been involved in the defence of the border town of Kobane from Islamic State in 2014-2015 - a lengthy battle that drew worldwide attention to the Kurdish forces' resistance against the jihadists and ended up winning them US air support.
The DFS have also freed six women and 15 children from Iraq's Yezidi minority who were being held as slaves by Islamic State jihadists in the village of Khirbet al-Rus on the southern edge of Minbij, the observatory said.
The jihadist organization enslaved or killed thousands of Yezidis after capturing the Sinjar region of Iraq, one of the religious minority's main homes, in June 2014.
Islamic State views the Yezidis, whose ancient religion is linked to Zoroastrianism, as infidels.
Yezidi men who were captured were reportedly killed or required to convert to Islam, while the group boasted of distributing Yezidi women and girls as concubines to its fighters.
Sinjar was recaptured by Kurdish forces in November last year. Several mass graves, thought to hold the remains of slain Yezidi civilians, have been found there since.