The area in northern Aleppo province, controlled by the Turkish army and allied Syrian forces, regularly sees deadly attacks and targeted killings.
The Observatory blamed the Daesh group for Sunday's attack, saying it was possibly in retaliation for a surge in operations by Turkish and Turkish-backed forces against Daesh cells in the region.
The Bab Al Salam crossing was a key entry point for cross-border humanitarian aid into northern Syria, but it was closed earlier this month after Russian pressure at the United Nations Security Council.
The Turkish official news agency Anadolu reported that a car bomb in the Azaz area, near Bab Al Salam, had killed five people and wounded 85. It accused the Kurdish YPG militia of being responsible.
In April, at least 46 people including Turkish-backed rebel fighters were killed when a fuel truck bomb went off in Afrin, a city controlled by Ankara's proxies.
The Syrian conflict has killed more than 380,000 people since 2011 and forced more than half of the country's pre-war population from their homes.