At least five people were killed and 20 injured on Wednesday when a bomb exploded outside a theatre in the southern Russian city of Stavropol just before the start of a Chechen dance show, investigators said.

Investigators opened a criminal case under terrorism laws, the Prosecutor General’s Office said in a statement.

The ethnically Russian Stavropol region, which borders the violence-racked Muslim republics of the North Caucasus, has been hit by Islamist attacks in the past, but not in recent years.

Islamist rebels have vowed in recent months to expand their campaign of shootings and bombings to Russian cities. Suicide bombers on the Moscow metro in March killed 40 in the worst attack on the Russian heartland since 2004.

The prosecutor’s office statement said the bomb, which contained explosives equivalent to 200-250 grammes of TNT, exploded 15 minutes before the start of a concert by a celebrated Vainakh dance troop from Chechnya.

The dance troop is closely associated with Chechen leader Ramzan Kadyrov, who has posed for photographs with the dancers.

“About 15-20 minutes before the start of the concert we heard an explosion. We saw the blast had practically flung aside the crowd that had gathered outside…about 100-150 people,” Rustam, an eyewitness, told Ekho Moskvy radio.

Two bodies covered in white sheets lay near the exit of the Stavropol Concert Hall, which was sealed off by police. RIA news agency quoted local hospitals as saying at least 40 were injured in the blast.

‘BRUTAL PROVOCATION’

“This is an unprecedented, brutal provocation,” said Stavropol Region Governor Valery Gayevsky, Interfax reported.

Russian President Dmitry Medvedev last year ordered that Stavropol Region be included in a new North Caucasus Federal District along with mainly-Muslim Chechnya, Dagestan and Ingushetia in a bid to tackle growing violence.

Medvedev’s new envoy to the district, former metals executive Alexander Khloponin, on Wednesday called an emergency meeting to discuss the bombing, RIA reported.

Stavropol city is 350 km (220 miles) northwest of Chechnya’s local capital Grozny. It has largely escaped Islamist insurgent attacks, but the surrounding region has seen some of the deadliest attacks in the long-running conflict.

Chechen rebels seized hundreds of hostages in a hospital in the Stavropol Region town of Budyonnovsk in 1995 and more than 100 died during the rebel assault and a botched Russian raid.

In the last major attack, seven Russian policemen and 12 gunmen were killed when special forces stormed houses to fight rebels holed up in a village near the city in 2006.

(Writing by Conor Humphries; Editing by Matthew Jones)