Over the past year the group have claimed to have killed several American and British citizens – including British aid workers David Haines and Alan Henning.

In more recent events, IS claimed responsibility for an attack in Texas, said to have been sparked by a cartoon contest featuring images of the Prophet Muhammad.

However, counter-terrorism experts have cast doubt over their claims by suggesting the group has a history of asserting involvement in events that have no link to them.

Meanwhile, it emerged yesterday that IS have “trained soldiers” in 15 states across America waiting to carry out deadly missions, according to an online warning from the group.

The current highest reward offered under the Rewards for Justice scheme is up to $25 million (£16.5 million) for information on Ayman al-Zawahiri, who was named as leader of al-Qaeda following the death of Osama Bin Laden in 2011.

It also offers up to $10 million (£6.5 million) for Abu Bakr al-Baghdadi, overall leader of IS and the jihadists’ self-declared ‘Caliph’.