At least 13 people are said to have been killed after Syrian military aircraft targeted rebel-held districts in the northern city of Aleppo for the third straight day.
The airstrikes were the latest in president Bashar Assad's air campaign aimed at driving the opposition out of Aleppo, Syria's largest urban centre and the country's commercial hub.
The opposition has controlled parts the city for more than a year.
Government war planes bombed Aleppo's rebel-held Shaar district, according to the UK-based Syrian Observatory for Human Rights. The Observatory, which relies on a network of activists on the ground, said at least two children were among those who died in the attack.
On Sunday, 76 people, including 28 children, died in air raids and the city was hit by another round of airstrikes on Monday.
The government assault on the opposition's northern stronghold comes just hours after UN Secretary General Ban Ki-moon demanded a cease-fire in the nearly three-year-old conflict in which more than 120,000 people have been killed, according to activists.
Mr Ban told reporters in New York that the situation in Syria has "deteriorated beyond all imagination" and insisted that the fighting must stop before political dialogue on Syria can start.
Peace talks between Syrian opposition and Assad's government are scheduled to begin on January 22 in Geneva.
However, violence in the country has surged in the past weeks as the warring sides try to claim or hold on to territory as a possible bargaining chip in the negotiations.
Civilians continue to pay the highest price in the conflict after even the most modest attempts at peace have failed.
On Monday alone, at least 150 people were killed nation-wide, the Observatory said. Most of the casualties were reported in and around Syria's largest cities, including in the capital, Damascus, Aleppo and the central city of Homs.
The high daily death toll coincided with the United Nations' £3.9 billion appeal to help displaced Syrians and their host countries as civil war is expected to rage on into 2014.
Millions of Syrians have been uprooted from their homes, with some 2.3 million fleeing into neighbouring countries and millions of others searching for shelter in safer parts of Syria.