Four people were killed and many more wounded on Wednesday in an attack on a bus packed with Israelis at a Black Sea airport in Bulgaria, which the government in Israel blamed on archfoe Iran.
Israeli Prime Minister Benjamin Netanyahu said "all signs point to Iran" after an official in Jerusalem said the bus carrying Israeli citizens at the Burgas airport was shot at and an explosive device thrown.
"Israel will respond forcefully to Iranian terror," Netanyahu said.
It would be the first attack against Israelis on Bulgarian soil.
The blast came on the anniversary of an attack on a Jewish community centre in Buenos Aires in 1994 that killed 85 people and wounded 300. The attack was blamed on Iran, which denied the charge.
An interior ministry press officer said in Sofia that three people were killed and over 20 wounded, while the mayor of Burgas said a fourth person had died and more 30 wounded.
The blast occurred at around 1400 GMT on a bus carrying Israelis who had flown in to Burgas, the second largest city on Bulgaria's Black Sea coast, setting off a fire that spread to another two buses, the ministry said.
News agency pictures showed huge plumes of black smoke rising over the airport, which the ministry said had been closed.
The two countries enjoy good relations and the Black Sea coast has become a popular holiday spot for Israelis.
"I was on the bus and we had just sat down when after a few seconds we heard a really loud explosion," one Israeli tourist Gal Malka told Israeli army radio. "The whole bus went up in flames," she said.
In Bulgaria, the BGNES news agency's correspondent at the airport cited unnamed official sources as saying the number of dead was at least seven.
"Apparently there was an attack on a bus carrying Israelis at Burgas airport in Bulgaria. A bus carrying Israeli citizens was shot at and an explosive device was thrown," said Ofir Gendelman, a spokesman for Netanyahu.
"What we know is that sometime close to 5:00 pm (1400 GMT) there was a flight that landed in Burgas, the passengers went onto a bus and then there was an explosion that we don't know the source of," said Ilana Stein, deputy spokeswoman for the Israeli foreign ministry.
"What we know is that there are casualties and probably not only injured but also dead. We know that some were Israelis but we don't know if all of them were," she said.
Two more buses caught fire but were quickly put out, the Bulgarian interior ministry said.
Between 40 and 50 passengers had got on to the bus at a parking lot outside the arrivals hall at the airport when the blast occurred, state-owned BNR radio said.
In January, Israeli public television reported that Bulgarian authorities had foiled a bomb attack targeting a bus chartered to take Israeli tourists to a ski resort.
The device was found on a bus that had been due to transport a group of Israeli tourists from the Turkish border to a Bulgarian ski resort, according to the report.
Bulgarian troops were deployed in several ski resorts frequented by Israeli tourists after the bomb was found, the report added.