Two ethnically-Chechen brothers suspected of carrying out the Boston Marathon bombings had no connection with Chechnya, the leader of the mainly Muslim region in southern Russia said Friday, blaming their upbringing in the United States.

"They grew up in America, their views and convictions were formed there. The roots of evil must be sought in America," the strongman leader of the North Caucasus republic, Ramzan Kadyrov, wrote on his Instagram account.

While expressing condolences to the American people, he nonetheless criticised US law enforcement authorities for killing the elder brother Tamerlan Tsarnaev, saying that "it would have been logical to detain him and hold an investigation."

"Evidently the special services needed a result at any cost to calm down society," he wrote.

Kadyrov earlier told the RIA Novosti news agency: "We do not know the Tsarnaevs. They did not live in Chechnya. They lived and studied in America. What happened in America is the fault of the American special services."

"It has become typical to link everything that happens in the world to Chechens, to blame the Chechens, even for the tsunami," complained the leader, known for his outspoken remarks.

The Russian-language social networking page of the younger brother Dzhokhar Tsarnaev, 19, records that he speaks the Chechen language and joined two groups for Chechens.

Tamerlan Tsarnaev, 26, who was shot and killed by police in Boston, featured links on his YouTube site to people he identified as "terrorists", but did not mention his Chechen origins.

Their uncle in the United States said the brothers were ethnic Chechens who were born in the neighbouring republic of Dagestan, where their father lives.