Two suspects have been arrested over the killing of Russian opposition activist Boris Nemtsov, who was gunned down last week near the Kremlin in a brazen assassination that shocked the country, the state security agency says.

“Two suspects were arrested today in connection with this crime. They are Anzor Gubashev and Zaur Dadayev, and the head of state has been informed,” the head of the FSB federal security service Alexander Bortnikov told state television.

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He said the two men were from the Caucasus region.

“The necessary operational and investigative procedures are continuing,” Bortnikov said.

The country was left reeling after Nemtsov was shot four times in the back while walking with his girlfriend along a bridge just a short distance from the Kremlin in the centre of Moscow on February 28.

The 55-year-old was a longtime critic of President Vladimir Putin and an anti-corruption crusader who served as Boris Yeltsin’s first deputy prime minister in the 1990s.

Putin called the killing a “provocation” that had brought disgrace upon the country, vowing that everything would be done to bring to justice those who committed a “vile and cynical murder”.

Following the announcement of the arrests the former head of the FSB and now lawmaker Nikolay Kovalev told the RIA Novosti agency that initial information showed the two men were merely paid hitmen.

“The key is to find out who ordered this assassination,” he said.

Opposition activist Ilya Yashin welcomed the development in the case but called for more information on the men’s identities.

“We hope the arrest … is not an error but the result of good work by security forces, but for now it is hard to say,” Yashin told Interfax news agency.

“Quite frankly the execution of the investigation had not inspired any optimism, but the fact that there have been arrests inspires some optimism.”