Blast was so powerful that several other buses at the site had also been damaged, Reuters reported.
Israeli Prime Minister Benjamin Netanyahu immediately blamed Iran for the blast and said Israel would respond.
“All the signs lead to Iran. Only in the past few months we have seen Iranian attempts to attack Israelis in Thailand, India, Georgia, Kenya, Cyprus and other places,” Netanyahu said in a statement.
“Eighteen years exactly after the blast at the Jewish community center in Argentina, murderous Iranian terror continues to hit innocent people. This is an Iranian terror attack that is spreading throughout the entire world. Israel will react powerfully against Iranian terror,” he said.
The explosion comes on the 18th anniversary of a 1994 bomb attack on the headquarters of Argentina’s main Jewish organization by an Iranian-backed Hezbollah suicide bomber, which killed 85 people.
Though Tehran has denied involvement, some analysts believe it is trying to avenge the assassinations of several scientists from its controversial nuclear programme, which the Iranians have blamed on Israel and its Western allies.
Israel has threatened air strikes against Iran’s nuclear facilities if diplomatic efforts fail to stop Tehran getting nuclear weapons, which it denies it is seeking.
Burgas airport was closed after the incident and flights were redirected to the airport of Varna, police said.