In the aftermath of Wednesday's bomb attack on Israeli tourists in the Bulgarian resort of Burgas, Israeli security experts are warning that Iranian agents and terrorist groups connected to Iran are planning further attacks around the world.

Israeli Prime Minister Benjamin Netanyahu blamed Iran for the incident, which by Wednesday night had killed seven people. Thirty-five others were taken to hospital including two pregnant women. Some of the injuries were said to be life-threatening.

The attack took place after a charter flight carrying 150 Israeli holidaymakers landed in the Bulgarian resort, which is popular with Israelis. A bomb exploded in the luggage compartment of a bus that picked up 40 passengers from the airport.

An eyewitness said that after clearing passport control the Israelis had put their luggage in the boot and boarded the coach. Minutes later it exploded. Two buses parked nearby carrying other tourists from the flight also caught fire.

Israeli and other western security officials have been warning in recent months of an increase in attempts by Iran's intelligence services and proxies in the Middle East - particularly Lebanon's Hizbollah - to attack Israeli targets. "The Iranians' frustration is growing," said one senior intelligence officer. "They see how nuclear scientists and senior generals are being assassinated on the streets of Tehran, and how key Hizbollah and Hamas operatives are killed mysteriously, and they see the hand of Israel in all this. They retaliate and hunt for the weakest links."

Mr Netanyahu said that all the signs led to Tehran: "Just in the last few months we have seen Iran's attempts to harm Israelis in Thailand, India, Kenya and Cyprus. Exactly 18 years after the attack on the Jewish community building in Argentina, the murderous Iranian terror continues to harm innocent people. This is an Iranian terror campaign that is spreading throughout the entire world. Israel will respond with great force."

Mr Netanyahu was referring to the 1994 attack on the Amia building in Buenos Aires, attributed to a joint Iran-Hizbollah operation, in which 85 people were killed.

Hizbollah was quick to deny any connection to the Bulgaria attack. An official spokesman said in Beirut that when the movement "avenges" its slain operations chief, Imad Mughniye, it would not be in so "cowardly" a method as killing tourists.

While Hizbollah has been implicated in other recent attempts to kill Israelis abroad, it could be telling the truth as it is embroiled in, and distracted by, the deepening civil war in Syria.

An Israeli security official said: "It doesn't really matter whether Hizbollah are directly involved. Iran has other minions and its own agents. In the end, they are all serving the same purpose."

Last month, Jonathan Evans, the head of MI5, gave a rare public lecture in which he said that a "return to state-sponsored terrorism by Iran or its associates, such as Hizbollah, cannot be ruled out as pressure on the Iranian leadership increases."

Israeli officials did not blame Bulgarian security for the attack. The two governments have grown closer over the last few years and signed a number of co-operation agreements, including over security. The relationship has improved as Israel's old ties with Bulgaria's rival Turkey have deteriorated.

"We have good co-operation with the Bulgarians," said one official. "It is impossible to put a guard on every charter flight and bus with Israeli tourists. Israelis want to lead normal lives and we can't prevent them from going on holiday abroad; we don't want to.

"The Iranians and their proxies have been trying so hard, in the end it was almost inevitable that they would find a weak spot and succeed."

This is not the first time attempt on Israelis in Bulgaria. A similar plan to was uncovered in January when a bus was loaded with suitcases filled with explosives. Then, however, the Bulgarian security service, working with their Israeli counterparts, had prior warning and averted the attack.

This month has already seen aborted attempts by Iranian agents to attack Israelis in Kenya and Cyprus. There have also been attempts this year in India, where the wife of an Israeli diplomat was wounded by a car-bomb, Thailand, Azerbaijan and Georgia. In most cases, co-operation between the Mossad and local security has been successful.

Israelis believe that the most efficient way of preventing attacks is not through guards but by using intelligence and co-operation between security services to prevent incidents at the planning stage. Co-operation between Britain and Israel is believed to have disrupted the plans of a Hizbollah cell that tried to operate in the UK.

Wednesday's murders are unlikely to be the spark that sets alight a general conflagration between Israel and Iran. Even if evidence of Iranian guilt is found soon, the justification for an Israeli attack on Iran is much deeper than any single attack on civilians. Any war with Iran will be based on the Iranians' nuclear capability - and it will not be Israel's private affair but also involve the US, Britain, Saudi Arabia and others.

Israel's security services will hunt down the perpetrators of Wednesday's murders as in the past. But it is yet another episode in Israel's attempts to fight its enemies, while its civilians try to lead as normal as possible lives.