A previously unheard of Russian militant group - claiming links to al-Qaeda - said it carried out the 3 April St Petersburg metro suicide-bombing, which left at least 14 people.
The self-titled Katibe al-Imam Shamil group (al-Imam Shamil Battalion) said in a statement posted on the Mauritanian Nouakchott News Agency (ANI) website that the attack had been carried out as retribution for Russia's military role in Syria.
The group also cited Russian military agendas elsewhere, in Chechnya, and Libya.
It also said that the attack had been carried out under the instructions of al-Qaeda chief Ayman al-Zawahiri, threatening to carry out further attacks if Russia continued its military intervention in "Muslim countries", further warning that Moscow would "pay in blood".
"This operation is only the beginning," continued the statement.
ANI is often used by West and North African jihadist groups to release statements.
The statement has also been circulated among al-Qaeda supporters on the Telegram messaging app. However, commentators have urged caution.
They have noted that while ANI has previously carried messages from al-Qaeda's African branch, it has not previously done so regarding al-Qaeda operations elsewhere.
The timing of the claim of responsibility - coming a number of weeks after the attack - has also attracted scrutiny, as out of sync with standard al-Qaeda practice.
At the time of publication Russia was yet to comment on the development.
Russia's military intervention in Syria has helped to turn the conflict in favour of the Bashar al-Assad regime.
Speaking in 2015, Russian President Vladimir Putin said that Moscow's intervention would also serve to keep "terrorists" from reaching Europe.
However, Russia's continued presence in Syria, has led to increased threats of retribution from both al-Qaeda and the Islamic State group, in addition to arousing the ire of restive groups in the North Caucasus.
During the 1990s and 2000s Islamist groups from the North Caucasus carried out frequent attacks in Russia, including a 2010 Moscow metro attack that killed at least 40 people.
However, since then attacks have largely been restricted to southern Russia.
Since 2011 hundreds of Russians have also travelled to Syria to join IS and the al-Qaeda-linked Jabhat Fatah al-Sham, a phenomenon that has further raised concern of terror attacks in the country.
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