Russian authorities have detained two men in connection with the murder of the opposition leader Boris Nemtsov.

The pair were named as Anzor Gubashev and Zaur Dadayev, both from the North Caucasus, a volatile region of southern Russia plagued by insurgency.

Alexander Bortnikov, head of Russia’s federal security service, said the investigation was ongoing and Vladimir Putin had been informed of the detentions, the government television network Rossiya-24 reported.

Nemtsov, 55, was shot four times in the back by a gunman in a passing car while walking close to the Kremlin on the evening of 27 February.

It is not clear whether either of the detained men is suspected of firing the shots that killed Nemtsov. The prime witness to the killing returned to Kiev this week. She told the media she was unable to identify who shot Nemtsov.

Ilya Yashin, an opposition activist, welcomed the detentions but called for more information to be released about the two men. “The execution of the investigation had not inspired any optimism, but the fact that there have been arrests inspires some optimism,” he said.

Nemtsov’s killing came two days before he was due to lead an opposition rally in Moscow. A memorial held instead was attended by tens of thousands of people.

Many believe the killing, which occurred in an area of high security near the Kremlin, would not have been possible without official involvement, and may have been an attempt to intimidate other government opponents.

Another opposition figure, Alexei Navalny, who was released from a two-week stint in jail on Friday for organising the initial anti-government rally, accused Russia’s “political leadership” of ordering a hit on Nemtsov.

“There will be no let-up in our efforts, we will give up nothing. This act of terror has not achieved its goal,” he said.

Friends said Nemtsov had been working on a report containing what he described as proof of Russian military involvement in eastern Ukraine.

Nemtsov was deputy prime minister in the 1990s in the government of Boris Yeltsin, which made him a tainted figure in the eyes of some Russians.

He had written a number of reports in recent years linking Putin and his inner circle to alleged corruption, and was one of the best-known politicians among Russia’s small and beleaguered opposition.

In 2013, he said as much as $30bn of the estimated $50bn funding for the Winter Olympics in Sochi had gone missing – a claim the Kremlin denied.

Nemtsov’s killing was condemned by world leaders, with the office of the French president, François Hollande, describing it as an assassination. David Cameron said the callous murder must be “fully, rapidly and transparently investigated”.

On Friday Nemtsov’s daughter Zhanna told CNN that her father died a hero and that the Russian authorities held political responsibility for the killing.

She said she had no confidence that those responsible for her father’s death would be brought to justice: “Russia has crossed the line after this murder and people will be frightened to express ideas contrary to the official standpoint.”

Putin has called the killing a “provocation”, vowing that everything would be done to convict those who committed a “vile and cynical murder”.