Heavy bombardment by forces loyal to Syrian President Bashar al-Assad killed at least 18 civilians on Monday in a residential district of the northern city of Aleppo, a monitor said.
"Regime forces fired on the Al-Shaar neighbourhood in Aleppo city's east, which is controlled by the opposition, and killed at least 18 civilians," said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights.
"A surface-to-surface missile hit the Al-Shaar neighbourhood. People started gathering, and that's when the army fired more missiles at the same area," he said.
Abdel Rahman said dozens of people were wounded and others were still trapped under the rubble.
Chaos reigned as screaming men carried wounded civilians from collapsing buildings.
"The civil defence came here to pull people out of the rubble, put out fires and save people," one emergency worker told AFP.
A man standing on the charred carcass of a car held his head in his hands as he stared into the lobby of a partly destroyed building littered with debris.
"This is a public market and all of these people were shopping. Every time he (Assad) suffers a defeat, he takes it out on civilians," a resident said.
Aleppo, once Syria's economic powerhouse, has been devastated by fighting since 2012.
It is now divided between government control in the west and opposition control in the east.
Much of Aleppo has been left in ruins as regime forces carry out aerial attacks and rebels retaliate, despite criticism of both sides from humanitarian organisations.
Further east along Syria's border with Turkey, four people were killed in twin car bomb attacks on the frontier town of Ras al-Ain, the Observatory and Syria's official news agency said.
The Observatory said that Kurdish security forces were among the dead.
SANA said that "terrorist suicide bombers... detonated a huge amount of explosives" just outside the town and that another four people were wounded.
Ras al-Ain, in Hasakeh province, was the site of ferocious fighting in 2013 between Kurdish militia and the Islamic State group before the former drove the jihadists from the town and its nearby border post.