At least seven people were killed in an attack against Israeli youth on a tour bus in Burgus Airport in Bulgaria.

Reports claim the attack was most likely carried out by a suicide bomber, who was either next to the bus or who got on the bus. According to nrg, a female suicide bomber stood next to the front of one of the buses and activated the bomb.
UPDATE:
@BarakRavid:

Bulgarian FM spoke with FM Lieberman and told him the investigation found the explosion was caused by a bomb hidden in the trunk of the bus

Israeli Prime Minister Bibi Netanyahu stated that “all signs point to Iran” and that Israel will respond fiercely.

@ruslantrad:

bTV showing photos from attack against Israeli tourists in Bulgaria

Shoshi, Israeli who witnessed the blast said:

We passed immigration and got onto bus #4 outside the airport. We placed our bags, and after two minutes bus #2 went up in flames. We were evacuated into a secure room.

Her son added:

Survivors from the bus had to jump out of the bus so as not to step on bodies. We saw the bus, one Israeli took a photo of it. We saw its trunk explode. We were quickly rushed into the terminal.

On his Facebook page, foreign minister Avigdor Liberman posted:

I just spoke with Bulgaria's foreign minister, Nikolay Mladenov, about the blast. Mladenov is heading to the location and will give me updates when he gets there.

A tweet from @MokedNews claims:

One of the killed in the Bulgarian attack – a Bulgarian tour guide who was with the Israeli tourists as the blast happened. (source: Bulgarian TV)

Excellent live blog coverage on Haaretz in Hebrew (more frequent updates) and in English.

Tweeps to follow for live updates:
@BarakRavid – diplomatic correspondent, Haaretz newspaper
@ruslantrad – Syrian-Bulgarian blogger & MENA analyst
@MokedNews – Israeli news feed