MAKHACHKALA, Russia (AFP) - One of the brothers suspected of carrying out the Boston bombings had made links with two figures in the Islamist anti-Kremlin insurgency in the Northern Caucasus, both of whom were killed by Russian security forces, a security source said Monday.

Tamerlan Tsarnaev, 26, who was himself killed during his capture by US authorities, was known to have been in contact with a Dagestan militant named Makhmud Nidal and also a militant of Canadian origin named William Plotnikov, a Russian security source in the Northern Caucasus said.

Plotnikov took part, like Tamerlan Tsarnaev, in boxing competitions in both Canada and the United States and the two men also had contacts on social networks, said the source.

Plotnikov was among seven militants killed in a shootout with Russian security forces in July 2012. It was not clear if they ever met on the territory of Dagestan itself.

According to the Moscow-based Novaya Gazeta newspaper, which also published details of Plotnikov's links to Tsarnaev, he was a 21-year-old ethnic Russian who had converted to Islam in Canada.

It said that Tsarnaev's name first became known to the Russian security forces when Plotnikov was arrested in 2010 in the Dagestan town of Izberbash.

He was later released.

Makhmud Nidal meanwhile was a known militant with whom Tamleran Tsarnaev was seen when he made a trip back to Dagestan from the United States in 2012, the security source said.

They were seen together four times, on each occasion at a mosque known for its Salafist tendencies in the Dagestani capital Makhachkala.

During his visits to the mosque he became the subject of some interest among fellow worshippers owing to an unusual interest in the study of Islam.

Nidal was killed in May 2012 in a "counterterrorist" operation in Makhachkala. After his death, Tsarnaev disappeared from the view of the Russian security services.

US and Russian security services are looking closely for signs Tamerlan Tsarnaev and his brother and suspected accomplice, Dzhokhar, had active links to the Islamist underground in the Northern Caucasus or acted on their own.

Dzhokhar Tsarnaev, 19, who was wounded in an escape attempt, has been charged with using a weapon of mass destruction and could face the death penalty if convicted in US federal court.