Notorious al-Qaeda leader Ayman al-Zawahri has warned the US of the "gravest consequences" if Boston marathon bomber Dzhokhar Tsarnaev, or any other Muslim prisoner, is executed. Tsarnaev, 22, has been sentenced to death for his role in the 2013 terrorist attack that killed three and injured more than 260 people at the finishing line of the Boston marathon.

The Kyrgyzstani-American citizen was named in a new online video message from al-Zawahri who also slammed the Islamic State (Isis) terrorist group and urged more so-called lone-wolf attacks on the west.

The Egypt-born Al- Zawahri became leader of the infamous jihadists in 2011, after US forces killed Osama bin Laden. Despite the relative decline of the terror group, hundreds of jihadists in Syria and Afghanistan still pledge their allegiance to the white-bearded veteran jihadi.

In the video al-Zawahiri said: "If the U.S. administration kills our brother the hero Dzhokhar Tsarnaev or any Muslim, it ... will bring America's nationals the gravest consequences".

He also encouraged Muslims to take captive as many Westerners as possible, especially those whose countries had joined the "Crusaders' Campaign led by the United States", with the possibility of an exchange deal for Muslim prisoners, say Reuters.

He continued saying that Western powers "are criminals and they only understand the language of force" in the video. The footage features images of Tsarnaev, who is caged in a Supermax" high-security prison in Florence, Colorado.

The Pentagon is trying to track the location of al-Zawahri who is believed to be in a remote mountainous region close to the Afghan-Pakistan border. In his last message, published in June, the 63-year-old warlord released a brief audio statement mourning the loss of Taliban leader Mullah Akhtar Mansour, whilst pledging allegiance to his replacement.

Tsarnaev carried out the Boston bombings alongside his older brother Tamerlan, who was killed in a confrontation with police on 19 April 2013. Despite being sentenced to death by legal injection, he may not be executed for years or even decades.

Al-Zawahri also says in the video that Daesh (Isis), who are expanding into the traditional al-Qaeda stronghold of Afghanistan, that there self-declared "caliphate" as being one of "severity and injustice."

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