The mother of the Boston bombing suspects was called by the FBI years ago to ask if her eldest son had been radicalised, she said in an interview with a British TV station broadcast on Tuesday.

Zubeidat Tsarnaeva told Channel 4 News that US federal agents had approached her about her eldest son Tamerlan Tsarnaev, 26, who was killed in a shootout with police on April 19.

Tamerlan's brother Dzhokhar, 19, has been charged over his alleged role in the attack on the Boston Marathon and remains in hospital with gunshot wounds.

Their mother told Channel 4 News: "They were monitoring him (Tamerlan) -- and I know that because I used to talk to them. They used to come to our house, like two, three times."

Speaking from Russia's southern region of Dagestan, she said an FBI agent had called and asked her: "Do you think that he could get involved into kind of like, any organisation -- you know, like radical organisation?"

In broken English, she insisted that her sons had nothing to do with the twin blasts at the Boston Marathon on April 15, which killed three people and injured some 200.

"What happened was a terrible thing," she said. "But I know that my kids have nothing to do with this. I know it. I am mother. I know my kids."

In another interview with Britain's Daily Telegraph newspaper, Tsarnaeva said she and her husband planned to bury Tamerlan in the United States.

"We are planning to leave for America this week," she told the newspaper. "We want to find a place to bury Tamerlan with Muslim rites in Boston, or somewhere nearby."

She described how Tamerlan had become increasingly devout in recent years and had encouraged her to start wearing a veil.

Her son had first aroused suspicions in the United States in 2008 and FBI agents met him "at least five" times afterwards, most recently a year and a half ago, she added.

CNN reported that Dzhokhar Tsarnaev told investigators from his hospital bed that his brother was the leader of the attacks.

Investigators are probing a six-month trip that Tamerlan made in 2012 to Russia's troubled regions of Dagestan and Chechnya, and whether he was radicalised or trained there.

US Senator Lindsey Graham said the FBI and Russian intelligence may have missed warning signs and made basic errors like misspelling Tamerlan Tsarnaev's name, allowing him to travel to Russia undetected.