Gunshots and explosions were heard as Russian security forces stormed an apartment in St. Petersburg Wednesday, August 17 as part of what police said was a counterterrorism raid, ABC News reports.

The Russian news agency RIA Novosti reported that counterterror police detained three men during the raid, quoting a spokesman from the FSB security service. The spokesman said those detained were suspected of belonging to terrorist groups based in the North Caucasus region, an area suffering from a long-running jihadist insurgency.

Beginning mid-morning, posts on social media and reports in local news outlets showed heavily armed police in helmets and body armor surrounding the apartment block on Leninskii Prospekt in St. Petersburg’s suburbs.

Witnesses told the Interfax news agency they had heard what sounded like shots, as well as loud booms that made the walls of the building vibrate.

Armed police cordoned off the building, with ambulances standing nearby; residents in surrounding buildings told Interfax and other local outlets police were not allowing them outside though they had not been evacuated.

Russia is on high alert for terror attacks. After the country’s military intervention in Syria, the Islamic State promised to retaliate with attacks on Russian soil, ABC says.

In recent weeks, Russian security services have been conducting anti-terror operations at increased tempo in the North Caucasus, where Moscow has been battling jihadist militant groups for well over a decade.