The suicide bombing of a Bulgarian bus packed with Israeli tourists has stoked fears of a deadly new phase in the long-running “shadow war” between Iran and Israel, with ordinary civilians now apparently replacing diplomats as primary targets.

U.S. and Israeli officials on Thursday pointed to similarities between the Bulgarian attack and three recently foiled plots also aimed at civilians, including a nearly identical plan to kill vacationing Israelis in Cyprus. While the identity of the bomber in Wednesday’s attack remained unclear, the earlier attempts have been tied to Iran or Hezbollah, the Lebanon-based militia movement closely aligned with the Islamic republic.

Even before the blast in Bulgaria, intelligence officials were seeing signs of a dangerous escalation in what had until recently been a campaign of covert, tit-for-tat strikes targeting diplomats and — inside Iran — nuclear scientists.

Earlier this month, Kenyan authorities arrested two Iranian men in connection with a plot to bomb several Western and Israeli businesses in that East African country. The suspects, identified by Kenya as members of an elite Iranian military unit, had brought with them more than 220 pounds of RDX, a powerful military explosive strong enough to destroy a large hotel.

As far back as January, the Israeli government has sounded warnings about a growing terrorist threat in Bulgaria, a country whose Black Sea beaches have become a popular destination for thousands of Israelis each year.

After Wednesday’s attack, Israeli officials were quick to blame Iran, but Israel did not release evidence linking Iran or Hezbollah to the incident. U.S. intelligence officials said they have not seen proof, though they did not dispute the link.

A series of Iran-linked plots in the fall and winter had mostly targeted diplomats and embassies. Iranian nationals and Hezbollah operatives had been implicated in attempted assassinations of Israeli, U.S. and Saudi figures in five countries. In one incident, Iranian operatives allegedly sought to hire Mexican gang members in a foiled plan to kill Saudi Arabia’s ambassador to Washington.

The attacks paused for several months during the spring, a lull that coincided with preparations for nuclear talks between Iran and the United States and five other world powers. But as the negotiations faltered in June, new plots surfaced, this time with civilians as primary targets.

In a plot eerily similar to Wednesday’s attack, authorities in Cyprus announced July 7 that they had detained a Lebanese man who confessed to entering the country to plan attacks on planes and buses used by Israeli tour groups.

The 24-year-old Lebanese man arrested by Cypriot police had traveled to the country on a Swedish passport, and he acknowledged under questioning that he was affiliated with Hezbollah, according to Israeli officials and Cypriot police reports. A search of his hotel room turned up documents detailing plans to blow up either a plane or tour bus. The material also revealed that the man had been collecting information about flight schedules of charter planes from Israel, as well as the routes of tour buses.

Matthew Levitt, a counterterrorism expert who is writing a book on Hezbollah-sponsored terrorism, said the new plots pointed to a tactical shift by Iran that suggested both a deliberate escalation and an acknowledgment of the difficulty of going after embassies and other heavily guarded installations.

“They’re going after softer targets,” said Levitt, a researcher with the Washington Institute for Near East Policy, a think tank.

The new plots “fit within the pattern of the shadow war” that pits Iran and its proxies against the West, he said. “But what is shocking is the fact that, while the modus operandi is the same, this time they succeeded.”

U.S. officials cautioned that there was, as yet, no firm evidence linking Iran or its allies to Wednesday’s attack in the Black Sea port of Burgas. In the latest incident, a suicide bomber, disguised as a tourist and carrying a fake Michigan driver’s license, managed to blend in with an Israeli tour group at the city’s airport before detonating his explosives, killing five Israelis as well as their Bulgarian bus driver and himself.

A succession of Israeli government officials blamed Iran and Hezbollah for the attack, citing unspecified intelligence. One of the country’s top former defense officials went further, directly linking the suicide bombing to the broader covert war with Iran. A spate of car bombings since 2010 killed four senior Iranian nuclear scientists and, in a separate phase of the covert battle, sophisticated cyberattacks have slowed Iran’s nuclear program.

“We are in a battle against Iran,” former Israeli national security adviser Uzi Arad said in an interview broadcast on Israel’s Army Radio. “We are an active side. We are not passive. . . . Any person with eyes in their head understands that this is what is behind us and ahead of us.”

Iran denied having any role in the Bulgarian attack, and its official media dismissed Israeli accusations as “ridiculous.”

Both Iran and Hezbollah have publicly blamed Israel and the United States for the assassinations of nuclear scientists and the assassination of the militia’s former security director, Imad Fayez Mughniyeh, who was blown apart in Syria in 2008 by a car bomb detonated by remote control.

Iran has also accused U.S. and Israeli operatives of launching sophisticated cyber attacks in recent years aimed at disrupting its nuclear program. The main attack involved a computer virus called Stuxnet, which crippled Iran’s main uranium enrichment plant.

While refusing to elaborate on specific programs or incidents, both Israel and the United States have acknowledged using an array of covert means to slow Iran’s nuclear program. Both countries and other Western nations believe the effort is aimed at providing Iran’s leaders with a nuclear-weapons capability. Iran contends that it seeks nuclear power only for peaceful, civilian applications.

The Obama administration said Thursday that it would continue to use of a variety of measures to persuade Iran to curb its nuclear ambitions. It also vowed to punish whoever was responsible for the deaths of Israelis in Bulgaria.

“We will work with and provide assistance to both Israel and Bulgaria in the effort to find out who was responsible for the attack in Bulgaria,” White House spokesman Jay Carney said, “and to hold accountable the responsible party or parties.”