The attack came as President Bashar Assad's forces have seized the momentum of the country's 3-year-old civil war ahead of presidential elections scheduled for June 3.
The Britain-based Syrian Observatory for Human Rights said the blast killed 43 people and wounded more than 80. Injured Syrians taken to hospitals in Turkey and later died are among the 43 killed, said Rami Abdurrahman, the head of the Observatory. The group relies on a network of activists on the ground.
An amateur video posted online showed women, men and children at the scene of the blast near the Bab al-Salameh border crossing in the northern province of Aleppo.
In another video posted online by activists, burned out cars are seen in the area near the crossing and the site of the attack, as people walk pass pools of blood, with clothes and other personal belongings scattered all around.
The Syrian conflict started as largely peaceful protests against Assad's rule and turned into civil war after some opposition supporters took up arms to fight a brutal government crackdown on dissent. Activists say more than 150,000 people have been killed and millions have been driven from their homes, seeking shelter in neighbouring countries and in safer parts of Syria.
Today's attack came as US Secretary of State John Kerry said the US and other nations supporting the Syrian opposition agree that the planned election is a "farce" because it excludes Syrians who have been displaced by fighting.
