Russian President Vladimir Putin says an explosion at a St. Petersburg supermarket on Wednesday was a terrorist act, and that another attack in the city had been foiled by the country's security service.

A bomb went off in a branch of the Perekrestok supermarket, wounding at least 13 shoppers in Putin's hometown, Reuters reports:

"Investigators have opened a criminal case into Wednesday evening's blast, which they say was caused by a homemade bomb packed with pieces of metal.

"Russian media reports said the bomb was hidden inside a rucksack in a locker where shoppers leave their belongings and said the person who left the bomb, described as being of 'non-Slavic appearance', had been caught on CCTV."

No group has taken responsibility for the explosion. A spokesman for Putin, Dmitry Peskov, would not say why authorities believed it to be terrorism, but said because the bomb was packed with shrapnel, that proved it "was a terrorist attack anyway." Investigators said the device contained 7 ounces of explosives.

A Kremlin spokesman told Reuters that the thwarted attack Putin referenced was the one earlier this month that targeted St. Petersburg's Kazansky Cathedral. The Federal Security Service said seven suspects linked to ISIS have been arrested in connection with that plot. Putin had called Donald Trump and thanked him for the CIA tip that led to the capture of the suspects.

On Thursday Putin said that the operation in Syria had reduced the threat of attacks in Russia, functioning as a preemptive strike against terrorists.

"What would have happened if those thousands (of terrorists) that I have just spoken about, hundreds of them had come back to us, trained and armed," he said in comments to Russian news agencies, The Associated Press reports.

In April, an explosion on the subway in St. Petersburg killed 16 people. Russian authorities said the bomber was a 22-year-old native of Kyrgyzstan.

Source: AFP