US officials says the Islamic militant group Hezbollah was responsible for the Bulgaria bus bombing which killed eight people, including six Israeli tourists, on 18 July.
Speaking to the New York Times, one of the officials said the accusations are backed up by intelligence reports, which show that the suicide bomber was "under a broad guidance" to hit Israeli targets.
The official added that the attack was guided by Iran, which was seeking "tit for tat" retaliation following the recent assassination of several Iranian nuclear scientists - an attack for which Iran blamed Israel and the US.
Israel has already blamed Iran for planning the bomb, which detonated outside Burgas airport on 19 July. Prime minister Benjamin Netanyahu, quoted in the Belfast Telegraph, said:
"The time has come for all countries that know the truth to speak it. Iran is the one behind the wave of terror. Iran is the No 1 exporter of terror in the world. A terrorist state must not have a nuclear weapon.
"The most dangerous country in the world must not possess the most dangerous weapon on earth."
Netanyahu vowed that the country would "continue to pursue the attackers and exact a heavy price from those who sent them," according to the report. He accused Hezbollah of being the "long arm" of Iran.
Bulgarian officials have so far refrained from pointing fingers at anyone as an investigation is under way. Bulgarian authorities released CCTV footage of the suspected suicide bomber on Thursday.
Iran denies responsibility for the attack. "Iran's position is to condemn all terrorist acts in the world. The Islamic republic, the biggest victim of terrorism, believes terrorism endangers the lives of innocents... is inhumane and so strongly condemns [it,]," foreign ministry spokesperson Ramin Mehmanparast was quoted by al-Arabiya as telling Arabic language television channel al-Alam.