Israeli Prime Minister Benjamin Netanyahu has blamed Iran for the bombing of a bus which killed at least seven Israelis in a Bulgarian resort.
He said "all signs point to Iran" and said Israel would "respond with force".
It is not clear whether the attack was a suicide bombing outside the Sarafovo airport or whether explosives had been packed onto the bus and then detonated. The Bulgarian Interior Ministy confirmed the explosion had been a terrorist attack.
Local media in the resort of Burgas claim the bus was packed with Israelis, possibly members of a young basketball team on their way to a training camp.
Reports suggest as many as seven people have been killed. Around 30 people have been taken to hospital to be treated for their injuries, while the airport has been closed in the wake of the attack. It is feared three pregnant women were among those injured.
The explosion is thought to have occurred at around 6pm local time (4pm BST). A flight from Tel Aviv had arrived around one hour earlier. All flights from Israel to Bulgaria have now been suspended.
The bus is believed to have been an airport shuttle carrying the holidaymakers. Around 40 people were thought to be on the bus.
An Israeli eyewitness told Voice of Israel Radio she had been among the group of holidaymakers.
She said after passing through passport control the Israelis had been moved on to buses.
"We put the bags in the trunk, and after a few minutes, the bus exploded in flames. Then we returned to the terminal, and now we are trying to determine who is missing and who is dead.
"People who survived the explosion escaped from the window, and could not step over the bodies. The bus was shattered on both sides.”
Eyewitness Aviva Malka told Israeli Army Radio she had escaped through a hole ripped in the bus and had seen bodies and "many people injured".
Cypriot authorites last week arrested a Swedish national believed to be a Hizbollah member suspected of planning an attack on Israelis.
Burgas is the second largest resort on the country's Black Sea coast. It is popular with Israeli tourists and is home to a Chabad centre. Israeli teens are known to flock to the resort for their summer holidays.
Israel has previously warned Bulgaria of the threat posed by Hizbollah. Bulgaria has a Jewish community of around 7,000.
In January a suspicious package was found on a bus carrying Israeli tourists from Turkey to Bulgaria, leading Israel to request additional security for its tourists travelling in the country, according to reports at the time in the Bulgarian press.
The Bulgaria attack coincides with the 18th anniversary of the fatal bombing of a Jewish community centre in Buenos Aires, Argentina, which killed 85 people and wounded hundreds.
Israel's emergency aid organisation Magen David Adom immediately sent volunteers to Burgas to assist with medical treatment.
Bulgaria's EU Commissioner Kristalina Georgieva said she was "shocked and saddened by the news about the explosion".