The suicide bomber launched an attack in the city of Afrin near a military base run by fighters supported by Turkey, the Syrian Observatory for Human Rights said.
The observatory, based in Britain, did not have a casualty toll for that attack.
But its head, Rami Abdulrahman, said two people were killed earlier on Thursday in two other blasts elsewhere in northern Syria.
One was in a marketplace in the town of Al Bab, in an area of war-torn Syria that in recent years has turned into an informal Turkish protectorate.
Dozens of people were wounded in the explosion, Mr Abdulrahman said.
Before that, one person was killed and several wounded when a car bomb exploded in the town of Azaz.
It was not immediately clear if the three explosions were linked.
All three towns are in the northern province of Aleppo, and under the control of Turkish forces and their Syrian allies.
Turkey and its allies have seized control of territory inside Syria over the course of several military operations launched since 2016.
The war in Syria has killed about 500,000 people and spurred the largest conflict-induced displacement since the Second World War after it broke out in 2011.