Driven from its self-styled caliphate in Iraq and Syria, ISIS is
down but not out, Reuters reported.
Where once they confronted armies, the extremist Islamist group's adherents have now staged hit-and-run raids and suicide attacks. In some cases, the group has claimed responsibility for atrocities, including the bombings of churches and hotels in Sri Lanka that killed at least 359 people.
Its involvement is not always proven, but even if the link is ideological rather than operational, ISIS still poses a security threat in many countries.
After defeat by US-backed forces, ISIS has reverted to the guerrilla tactics it was once known for.
Sleeper cells have regrouped in provinces including Diyala, Salaheddin, Anbar, Kirkuk and Nineveh, where they carry out frequent attacks, including kidnappings and bombings aimed at undermining the Baghdad government.
In February, two people were killed and 24 wounded when a car bomb went off in Mosul, once the group's Iraqi capital.
The Pentagon said in January that IS was regenerating faster in Iraq than in Syria. Analysts estimate that about 2,000 active combatants now operate in Iraq.
After serious military setbacks, ISIS slipped into the shadows, staging suicide bombings and ambushes.
ISIS has carried out bomb attacks in towns and cities of northeast Syria in recent months, including some targeting US forces.
Syrian Kurdish forces, which control the region and crushed the jihadists with US help, have sounded the alarm about the group's new tactics.
They believe sleeper cells have mushroomed across eastern Syria and expect guerrilla attacks to escalate. They also warn of the risk posed by holding thousands of militants in prison camps.
ISIS fighters still hold some ground in Syria's remote central desert, where they have staged attacks in recent days.
ISIS militants have carried out deadly bombings and shootings in Saudi Arabia against security forces and minority Shiite Muslims, after the authorities crushed an al Qaeda insurgency over a decade ago.
Saudi security forces said they foiled an attack by four ISIS militants north of Riyadh on Sunday, and arrested 13 others on Monday in connection with planning other attacks.
The security forces also raided a house they said the militants were using as a bomb factory, and seized suicide vests, homemade bombs, rifles and ISIS publications.
On Tuesday, the interior ministry said it had executed 37 people "for adopting extremist terrorist ideologies and forming terrorist cells to corrupt and disrupt security as well as spread chaos and provoke sectarian strife".
Kamran Bokhari, a director at Washington-based think-tank the Center for Global Policy, said ISIS does exist in the kingdom but the Saudi security forces and intelligence service are "pretty much on top of things".
"At end of the day, from an ISIS point of view, Saudi Arabia is the grand prize," said Bokhari, because of the kingdom's oil wealth and its prominent position in the Islamic world.
Michael Stephens, research fellow for Middle East Studies at London's RUSI think-tank, said the Saudi security forces are tracking a few hundred people, including some who have been to Syria, but there is no evidence that they have become operational.