Members of Saudi Arabia’s Shia minority are forming civil defence groups after twin suicide bombings over the past two Fridays — claimed by the Sunni jihadi Islamic State of Iraq and the Levant (Isis).

The attacks, which killed about 25 people, mark an escalation of Isis activity in the oil-rich kingdom, with the group saying it wants to rid the Arabian peninsula of Shia Muslims. Last November, Isis gunmen killed seven worshippers at a Shia shrine in al-Ahsa, in eastern Saudi Arabia.

The worst sectarian atrocities in the kingdom’s modern history have prompted the formation of civil defence groups among the country’s 10-15 per cent Shia minority, an “unprecedented development”, according to Toby Matthiesen, a research fellow at the University of Cambridge.

Activists say the self-defence groups have detained people on suspicion of planning to carry out further attacks, handing them over to security forces.

“These are volunteers who have seen a security vacuum and have to find a way to defend themselves,” said Hamza al-Hassan, an exiled Saudi activist based in London. “This is self-defence — it is not a political issue.”

Saudi authorities have strengthened security in Eastern Province — where most of the country’s Shia population live — boosting the number of checkpoints and enhancing inspections on the causeway to Bahrain. Analysts close to the government expect a further tightening of security akin to the crackdown on domestic jihadis that quashed the al-Qaeda insurgency of 2003-06.

Many Shia feel that more needs to be done. Some within the community have called for the arming of groups, sparking fears that self-defence civilian forces could form the first step towards militias such as the “Hashd Shaabi” in Iraq — so-called popular mobilisation forces to stem the advance of Isis, which have also been accused of carrying out atrocities against Sunni civilians.

The formation of such groups has alarmed the Saudi establishment, and some “volunteers” have already been detained by the authorities.

Their creation comes against a background of increased sectarian strife. While the ruling family and pro-government clerics have condemned the suicide attacks, activists have criticised the authorities for tolerating an increasingly divisive media and inflammatory statements by radical Sunni clerics.

Some school textbooks and conservative Wahhabi teachers compound discrimination by describing Shia Muslims as outside true Islam. A pejorative Sunni term for the Shia, “rawafid” (or “rejectionists”), is gaining currency in debate on social media.

Since the 2011 political revolutions, unrest in Bahrain and Iran’s support for the regime in Syria have sparked an upsurge in anti-Shia sentiment. The emergence of Isis as a defender of the Sunni world against transnational Shia expansionism has fostered growing support among many youths who see the group as one of the Arab world’s rare military successes.

Saudi Arabia’s war on Zaydi Shia rebels in Yemen has compounded these purist ideals by fostering a jingoistic euphoria in the kingdom as the state acts against perceived encroachment of Iran into Arab capitals. Isis’s emerging strategy appears to be to tap into that well of sectarianism — seeking to recruit from the growing ranks of disaffected Saudi youth.

“These appear to be young Saudis who are not travelling abroad, but are willing to carry out attacks at home,” said Mr Matthiesen, who fears further attacks. “Rather than going after foreigners in well-defended compounds, they are blowing up fellow Saudis, who happen to be Shia.”