Update (3:19 p.m. EDT): The White House has also issued a statement, via Breaking News:

I strongly condemn today’s barbaric terrorist attack on Israelis in Bulgaria. My thoughts and prayers are with the families of those killedand injured, and with the people of Israel, Bulgaria, and any other nation whose citizens were harmed in this awful event. These attacks against innocent civilians, including children, are completely outrageous. The United States will stand with our allies, and provide whatever assistance is necessary to identify and bring to justice the perpetrators of this attack. As Israel has tragically once more been a target of terrorism, the United States reaffirms our unshakeable commitment to Israel’s security, and our deep friendship and solidarity with the Israeli people.

Update (2:59 p.m. EDT): Breaking News has a copy of the Bulgarian Foreign Ministry's statement on the blast. The first paragraph includes specifics on the casualties:

According to preliminary data, six people were killed and over 30 were injured in an explosion on a tourist bus at the airport in Sarafovo. On-site explosion killed five, and one died later in hospital. 32 people were hospitalized, of which three are in intensive care.

Update (2 p.m. EDT): Israeli defense minister Ehud Barak said the explosion was "clearly a terrorist attack," CNN reports. "He said the attack was probably initiated by a group under the auspices of 'either Iran or other radical Muslim groups,' and named Hezbollah and Hamas as likely suspects."

Meanwhile, Reuters and AP have both pushed their reports of the death toll up to four, while Haaretz (which has re-posted that briefly deleted quote from Netanyahu) still reports seven. The Israeli news site is also reporting that one of the dead was a local tour guide.

Update (1:14 p.m. EDT): The AP has more from Netanyahu, quoting him saying "all signs point to Iran" and comparing the attack to similar recent incidents in India, Georgia, Thailand, Kenya and Cyprus. "This is an Iranian terror attack that is spreading across the world," he said, according to the wire. "Israel will react strongly to Iran's terror." Haaretz, meanwhile, has removed its quote from Netanyahu. It's also reporting now that seven have died. BBC, Reuters, and AP are still reporting three.

Update (1:04 p.m. EDT): Israeli Prime Minister Benjamin Netanyahu publicly blamed Iran for the attack, telling Israeli news site Haaretz, "Iran is responsible for the terror attack in Bulgaria, we will have a strong response against Iranian terror." 

Update (12:46 p.m. EDT): The Associated Press has some new casualty figures, reporting 27 injured, in addition to at least three dead.

Update (12:05 p.m. EDT): Witnesses told Israeli television that someone boarded the bus immediately before the huge explosion, according to both the Associated Press and the BBC reports. Flights into Burgas are being diverted to Varna, Reuters reports.

Update (12:03 p.m. EDT): Sofia is now calling the blast a terrorist attack, citing Bulgarian police, and saying that 18 to 20 people were injured in addition to the three to five killed. It says the tourists had boarded the bus, bound for the Black Sea resort of Sunny Beach, after arriving on a flight from Tel Aviv.

Original: A bus carrying Israeli tourists exploded at an airport in the Bulgarian city of Burgas, killing three people and closing the airport, the Bulgarian Sofia News Agency reports, with BBC confirming the news via Bulgarian officials. The details are still slim, and the major Western news services haven't confirmed the cause, but Sofia's initial one-sentence report says three people were killed as the bus exploded in the airport parking lot. A report on the Bulgarian newswire BGNES puts the number of dead at five. ZeroHedge's Tyler Durden cites the Bulgarian news site Segabg, which reports a bomb in the trunk of the bus exploded, and that 40 people were aboard for the blast at 5:30 p.m., local time. 

[Inset photo via AP]