The Bulgarian President Rosen Plevneliev was quoted by the national news agency as saying that he received no advance warning of an expected attack when he held a meeting with Israel's intelligence agency, Mossad, last month.
He said that the two countries' intelligence agencies share a good relationship and that he would have acted on any lead.
US President Obama called Israeli Prime Minister Netanyahu earlier today to offer his condolences.
– Readout of president obama's call with Israeli prime minister
The President called Israeli Prime Minister Netanyahu today to offer his condolences and assistance following the terrorist attack targeting Israeli tourists in Bulgaria. The President strongly condemned this outrageous attack that killed and wounded innocent Israelis and Bulgarians, including Israeli children. The President pledged to stand with Israel in this difficult time, and provide whatever assistance is necessary to identify and bring to justice the perpetrators.
– Shadow Foreign Secretary Douglas Alexander
Today's terrorist attack must be condemned and the perpetrators brought to justice. Although the full details of the tragedy have not yet emerged, our immediate thoughts are with the families and friends of those killed and injured in today's attack.
Barack Obama added his name to world leaders who have condemned the deadly attack on a bus carrying Israeli tourists in Bulgaria.
"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," Obama said in a statement, calling the attack "completely outrageous."
– Foreign Secretary William Hague
I condemn today’sterrorist attack on a bus carrying Israeli tourists at Bourgas airport inBulgaria. The full tragedy of the attacks is not yet clear, but I would like to express my condolences to the friends and family of those injured and killed
Six people have been killed in the blast that ripped through a bus carrying a group Israeli tourists. It is now believed that the explosion was caused by a bomb in the bus itself. Bulgarian Foreign Minister Nikolai Mladenov said: "The explosion was caused by a bomb in the bus."
In a statement, the foreign ministry said five people were killed and one died later in hospital. Thirty two people in total were taken to hospital with injuries.
White House spokesman Jay Carney said that the US condemns the attacks in Bulgaria and said the people of America stand with the people of Israel and Bulgaria, he added, "of course our commitment to Israel's security remains unshakeable."
The United States Ambassador to the UN has tweeted reaction to the blast in Bulgaria:
The terrorist attack against Israeli tourists in Bulgaria today is outrageous, an act of supreme cowardice.
We extend our condolences to the families of the victims of this heinous act and stand with the people of Israel & Bulgaria.
Bulgarian Foreign Minister Nikolai Mladenoc says that the airport explosion was caused by a bomb on a bus.