At least 52 people were killed after Syrian army forces artillery strike hit an anti-regime protest in Deir Ezzor, opposition activists said on Tuesday.
According to initial reports, forces loyal to Syrian President Bashar Assad struck a residential area of the city, located in the country's east, causing several buildings to collapse.
The incident took place after, on Monday, UN Secretary-General Ban Ki-moon's office indicated that violence in Syria was getting worse as the military steps up assaults on civilian centers and the opposition increasingly turns to coordinated attacks on Syrian government forces.
"The secretary-general expresses his deep concern at the dangerous intensification of armed violence across Syria over the past several days, and the grave danger facing civilians in areas under fire," Ban's press office said in a statement. "The bloodshed and fighting must stop at once."
It said unarmed UN observers in Syria have reported an increased level of armed clashes between government and rebel forces.
"The government's intensive military operations, including the shelling of Homs and reportedly other population centers, as well as firing from helicopters on Talbiseh and Rastan, are resulting in heavy civilian casualties and human rights violations," the statement said.
"The secretary-general underlines the importance of unimpeded access by UNSMIS to Al-Haffa, amid reports of a build-up of government forces around the town."
The UN observer force, known as UNSMIS, has also been observing "planned and coordinated attacks (by rebels) on government forces and civilian infrastructure in multiple locations."
"The violence as a whole is intensifying amid the shifting tactics," Ban's statement said. It added that the secretary-general urged both sides to comply with their commitment to peace which they made when they accepted international mediator Kofi Annan's six-point peace plan earlier this year.
"The secretary-general condemns this escalation of armed violence, in particular the shelling of population centers and attacks against civilian infrastructure by all sides, which impairs delivery of essential services and exacerbates the humanitarian crisis," the statement said.
Ban also urged "all countries with influence on either side to convince them to put the welfare of Syrians first, to pull back from the brink and to reflect on the devastating consequences that increasing violence is having for the people of Syria, the country and the region."
U.N. observers, tasked with monitoring Annan's April ceasefire deal, which never took hold, have instead been cataloguing mass killings, bombings and clashes in which many hundreds of Syrians have died.
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