Syrian rebel groups, including hardline Islamist militias, have been shelling a Kurdish neighbourhood in northern Syria for the past day, killing at least 16 civilians, including nine children, activists reported on Monday.
The rebel groups shelling Sheikh Maqsood in Aleppo city include al-Qaeda's al-Nusra Front. The group is excluded from the current Syrian ceasefire, which is shaky but holding in its second week.
"This is a serious violation of the ceasefire," said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights.
The shelling in Sheikh Maqsood, controlled by the Kurdish People's Protection Units (YPG), began on Sunday, reports said. There were reportedly Arab citizens among the casualties.
Tensions are high in northern Syria between Kurds and some opposition groups, especially after the YPG took control of areas north of Aleppo in the period immediately preceding the ceasefire.
On Monday, Turkish artillery also shelled the region of Tel Rifaat at the outskirts of Northern Aleppo, which is mainly controlled by the Kurd-led Democratic Forces of Syria (DFS), the British-based Observatory said. The DFS is a coalition linked to the YPG in the northern and north-western parts of Aleppo province.
The shelling wounded at least one DFS fighter and inflicted heavy material damage in the area, it added.
In the meantime, rebels loyal to Al Nusra Front and their allies Jund al Aqsa launched a wide-scale attack against regime forces in the southern Aleppo region, scoring some advances in the area.
Rebels in the area had been weakened by Russian airstrikes, as Moscow launched an aerial campaign against those fighting President Bashar al-Assad.
Syria's main opposition group said they will attend upcoming peace talks this month in Geneva and its delegation would arrive this weekend.
Suheir al-Attasi, a member of the opposition's negotiating committee, told dpa the group wants to focus the talks on the formation of a transitional government "with full executive powers."
The UN plans to resume the talks on Wednesday, said Farhan Haq, a UN spokesman in New York, noting that UN Syria Envoy Staffan de Mistura would be ready to receive participants starting that day and would conduct preliminary consultations before substantive talks begin.
World powers hope the Geneva negotiations will initiate a political process to resolve the Syrian conflict, but the Syrian opposition's demand that al-Assad have no role in the country's future is likely to be a sticking point.
The last round of talks, started in the weeks before the truce, saw several delays in their start amid fraught negotiations about which parties would participate.
The ceasefire, brokered by the US and Russia and in place since February 27, is being observed "on the whole," with some "individual provocations and gunfire," Russia's Defence Ministry said in a statement on its website.
Russia has helped provide more than 600 tonnes of aid, including food and medical supplies, to Syrian population centres and intends to continue air-dropping such provisions, the statement said.
Meanwhile, in Syria's north-western Idlib province, at least 12 people have been killed and more than 10 injured in an airstrike, activists reported.
They said it was a Russian strike in Abu al-Duhur, an area over which al-Nusra Front has partial control.
The Britain-based Syrian Observatory for Human Rights said the strike targeted a centre selling fuel.
Near the capital Damascus, some 23 trucks carrying humanitarian aid entered areas in the besieged rebel-held eastern Ghotta region. The trucks were mainly carrying food and medical supplies to the residents of th cities of Hammouriyeh and Jisrine, the Observatory said.
The Syrian conflict started as peaceful anti-government protests in 2011. It has since spiralled into a multi-sided civil war that has claimed more than 250,000 lives and displaced at least 11 million people from their homes, according to estimates aid groups, activists and UN data from 2015.