The blast, two days after new President Hassan Rohani took office, hit a pipeline feeding a petrochemicals plant in the city of Mahshahr in Iran's southwest, home to most of its oil reserves and to a population of ethnic Arabs, known as Ahwazis for the main town in the area.
The Ahwazi Arabs are a small minority in mainly ethnic Persian Iran, some of whom see themselves as under Persian "occupation" and want independence or autonomy. They are a cause célèbre across the Arab world, where escalating ethnic and sectarian rivalry with Iran now fuels the wars in Syria and Iraq and is behind political unrest from Beirut to Bahrain.
Tehran dismisses any suggestion that discontent is rife among its Arab minority, describing such reports as part of a foreign plot to steal the oil that lies beneath its Gulf coastal territory. Iranian news agencies reported a fire on the gas pipeline last week but said its cause was unknown.
There has been unrest in the area for many years, and now some Ahwazis see themselves as part of a larger struggle between Shi'ite Iran and the Sunni-ruled Arab states across the Gulf, which back opposing sides in the Syrian civil war.
Although the overwhelming majority of Ahwazis are Shi'ites, some say they sympathize with the mainly-Sunni rebels fighting Syria's Iran-backed President Bashar al-Assad.
"Our land is occupied and the Syrian people are in the shadow of a dictatorial regime that serves Iranian interests in the region," said an Ahwazi activist speaking from inside the region. "If Bashar falls, Iran falls: that is the slogan of the Ahwazis," he said.
The Islamic Republic would almost certainly outlast Assad's downfall. But the slogan nonetheless shows how events in Syria are stirring a latent threat to stability in one of the world's most resource-rich corners: the Iranian province of Khuzestan, once known as Arabistan for its Arab majority.
An Ahwazi militant group said it had sabotaged the pipeline with homemade explosive devices, targeting Iran's economy in revenge for the authorities' mistreatment of ethnic Arabs and for Tehran's roles in Syria and Iraq.
"This heroic operation is a message to the Persian enemy that the national Ahwazi resistance has the ability and initiative to deliver painful blows to all the installations of the Persian enemy, inside Ahwaz and out," the Mohiuddin Al Nasser Martyrs Brigade, which has claimed responsibility for previous attacks on energy infrastructure, said in a statement.
The group threatened to intensify its activities in coordination with members of Iran's Kurdish and Baluch minorities, some of whom also complain of unfair treatment.
Arabistan was a semi-autonomous sheikhdom until 1925, when it was brought under central Iranian government control and later renamed, marking the start of what some Ahwazis describe as a systematic campaign to Persianise if not obliterate them.
According to the CIA Factbook, Arabs make up about 2 percent of Iran's population, suggesting there are around 1.6 million of them, a small minority in a country with a Persian majority and much larger Azeri and Kurdish communities, among others.
At their most ambitious, Ahwazis want an independent state stretching beyond the borders of Khuzestan, which is at the head of the strategic Gulf waterway and shares a border with Iraq.
Iraqi dictator Saddam Hussein's attempt to annex Khuzestan triggered the Iran-Iraq war of the 1980s in which a million people were killed. "Liberating" the Ahwazis was a slogan for Saddam and the Arab states that supported him.
In 1980, with Iraqi support, Ahwazi separatists took 26 hostages in Iran's London embassy. British special forces stormed the embassy after a six day siege; two hostages and five captors were killed.
Thousands of Ahwazis crossed into Iraq during the Iran-Iraq war and some were given land, but they are no longer welcome under the Shi'ite-dominated government that rose to power after U.S.-led forces invaded in 2003 and toppled Saddam.
Exploited by avowed secular Arab nationalists like Saddam, Ahwaz is now being woven into the sectarian narrative revolving around the Syrian conflict, which has polarized Sunnis and Shi'ites. Ahwazis are overwhelmingly Shi'ite, but in recent years there has been some conversion to Sunni Islam among them.
"I converted for political reasons and I think most are like that," said the activist contacted by Reuters in Khuzestan, who decided to become Sunni during a trip to a Shi'ite shrine in the Iranian city of Mashhad after hearing several ethnic Persians travelling on the same train mock Arabs.
Iran's Deputy Minister for Arab and Foreign Affairs Hossein Amir Abdollahian told reporters in Kuwait there were no Sunnis in Khuzestan. Nevertheless, Sunnis across the Arab world have taken up the Ahwazi cause with zeal.
From a stage in the Iraqi province of Anbar, where Sunnis have rallied for months against a Shi'ite leadership they denounce as a stooge of Iran, lawmaker Ahmed al-Alwani roared: "We tell our people in Ahwaz: we are coming!"
In Bahrain, whose Sunni monarchy blames Tehran for fomenting protests by the Shi'ite majority on the island since 2011, a street in the capital has been renamed "Arabian Ahwaz Avenue".
A bearded presenter on Saudi-based hardline Sunni pan-Arab TV channel al-Wesal burst into tears recounting the sufferings of the Ahwazi people: "We must stand with them as Muslims! They are calling us," he said after composing himself.
One battalion of the rebel Free Syrian Army is called the "Ahwaz Brigade", although the group says there are no foreign fighters in its ranks.
"We have relations with different factions of the (Syrian) rebels," said Habib Nabgan, the former head of a coalition of Ahwazi parties whose armed wing carried out last week's pipeline attack.
"They need information, which we give them, and we need some of their expertise, so there is cooperation and that is developing," he told Reuters via telephone from Denmark, where he took refuge in 2006.
The use of Ahwaz for sectarian and Arab nationalist agendas has served to justify repression by Iranian authorities, which say they face a foreign plot to control the country's natural resources. Tehran has accused Britain, Israel and Saudi Arabia of provoking unrest in Khuzestan.
Although the bulk of Iran's 137 billion barrel oil reserves lie beneath the soil of Khuzestan, most Ahwazis struggle to scratch a living off the land they lay claim to.
"We get nothing from the oil and gas fields except smoke (from the refineries)," said activist Taha al-Haidari, in footage filmed secretly in prison before he was executed along with two of his brothers and a friend.
They were arrested after taking part in a protest in 2011 and convicted of "enmity against God" and "corruption on earth", having confessed under duress to murder and being members of an armed separatist group, one of them said in the video.
The authenticity of the tape, which activists said was smuggled out of jail, could not be independently verified.
Iran dismisses Ahwazi grievances and says reports of their mistreatment are mere propaganda, often pointing out that a former Iranian minister of defense was an ethnic Arab.
A document purporting to be a secret government directive leaked in 2005 described a policy to dilute the Arabs of Khuzestan by displacing them and encouraging others to settle there. The letter, which authorities said was forged, ignited protests that were put down by force, leaving at least 31 dead, according to rights group Amnesty International.
As the anniversary of that crackdown approached in 2011, Ahwazi activists began calling for a "Day of Rage" in the spirit of popular anti-government revolts in Egypt and Tunisia.
Protests broke out but were quelled by authorities who have since rounded up dozens of Ahwazi activists, at least five of whom are currently awaiting execution on terrorism-related charges, rights groups say.
Ahwazi groups are divided over whether to seek independence or devolution of power within a democratic, federal Iran.
"We have a right to seek independence, but is that possible at this time? I don't think so," said Abu Khaled, a member of the biggest Ahwazi federalist party, speaking in Dubai.
"We have to be pragmatists, or else we will be a part of history, like the Red Indians (Native Americans)".
(Editing by Peter Graff)