Israeli officials say that at least seven people died in a bomb explosion on a bus carrying Israeli tourists in the city of Burgas, Bulgaria.
About 30 people were also injured when the bus exploded at Burgas airport, by the Black Sea.
Witnesses told Israeli TV that someone boarded the bus and a huge explosion immediately followed.
Israeli Prime Minister Benjamin Netanyahu later accused Iran of being behind the explosion.
“All the signs lead to Iran,” he said in a statement. “Israel will respond forcefully to Iranian terror.”
The Israeli foreign ministry said: “There are six bodies on the scene – one critically wounded died at the hospital and two seriously injured are in intensive care. Thirty more people are being treated.”
It said the bus was carrying tourists from a charter flight that arrived from Israel.
Wednesday’s blast came on the 18th anniversary of a deadly attack on a Jewish community centre in Argentina. Israel blamed Iran for that attack – a claim denied by Tehran.
US President Barack Obama described the bombing as a “barbaric terrorist attack”.
Israeli officials said passengers from a Tel Aviv-Burgas flight boarded the bus shortly after 17:00 local time (14:00 GMT).
“I was on the bus and we had just sat down when after a few seconds we heard a really loud explosion,” Gal Malka told Israel’s army radio.
“The whole bus went up in flames,” she said, adding that the explosion took place near the front of the bus.
Bulgarian journalist Dobromir Dovkacharov, who arrived at the scene about 30 minutes after the blast, said: “I saw three buses completely burnt out – just the metal bars were left.
“There were crowds of people around, very distressed. One man said he saw decapitated heads. Others spoke of body parts flying through the air,” Mr Dovkacharov said.
Burgas airport has now been closed and flights are being diverted to Varna.
Two Israeli planes with emergency services personnel and diplomats on board are due to arrive in Burgas shortly. The officials will be assisting the injured and help identify the bodies.
Bulgaria is a popular tourist destination for Israelis.
However, in January there were reports that Israel had asked Bulgaria to tighten security for Israeli tourists travelling by bus.
This followed a reported discovery of a suspicious package found on a bus with Israeli tourists travelling from Turkey to Bulgaria.
Israeli tourists have been targeted in attacks in a number of countries around the world.