At least 67 people were killed and 185 wounded on Wednesday in an attack claimed by Islamic State in Qamishli, north-eastern Syria, medical sources in the Kurdish-controlled town said.
Rescue teams are still searching for victims under the rubble of the buildings hit by the attack, the sources added.
Earlier reports said that 55 people were killed and more than 160 wounded in what activists said was a powerful blast that rocked Qamishli, which is near the Turkish border.
"The number keeps increasing as lots of victims are still buried under the debris. Many of the injured are listed in critical condition," Qamishli National Hospital director Omar al-Qaqoub said.
Kurdish news agency Rudaw said the explosion was the result of a double attack. The first was caused by a truck loaded with explosives that targeted a centre run by the Kurdish police force.
A second blast hit a street in the east of the town that is home to the local defence administration, Rudaw said.
Rami Abdel-Rahman, the head of the Syrian Observatory for Human Rights, a monitoring group, said the resulting blast was the worst seen in Qamishli.
Syrian state television meanwhile aired footage showing huge damage reportedly from the area where the blast took place.
In the footage, panicking people are seen rushing to help those trapped under the rubble. Women are also seen running and shouting for help.
Hospitals across Qamishli have put out urgent calls for the donation of all blood types in the wake of the attack.
Qamishli, which is predominantly inhabited by Kurds, has been targeted by recent bombings, many claimed by Islamic State.
The al-Qaeda splinter group also said it was behind Wednesday's attack.
"An Islamic State fighter managed to reach a number of buildings of the Kurds via his explosives-laden truck, blowing himself up among them ... destroying their buildings ... killing over 100 and injuring dozens," according to an alleged online statement by the militant group.
It added that the attack was to avenge the killing of men, women and children in Islamic State-controlled Minbij city in northern Syria.
The authenticity of the statement could not be verified by dpa.
In May, Kurdish-led Syrian forces, backed by US-led airstrikes, started a major attack to dislodge Islamic State from Minbij and have since gained ground against the radical group.
The Minbij onslaught is aimed at cutting Islamic State's last link with the outside world and isolating its de facto capital in Syria, al-Raqqa, which lies south-east of Minbij.